Upstream Wall Research Articles

Detached eddy simulation of traffic-induced air pollution in an urban highway with complex surrounding morphology

Air pollution caused by traffic is a major contributor to unhealthy ambient air quality in the vicinity of urban highways and puts the health of residents and pedestrians at risk. Therefore, it is imperative to examine local pollution reduction methods. The present paper is concerned with Detached Eddy Simulation (DES) of an urban area with high concentration of air pollutants. As an example of a complex surrounding morphology, the studied domain encompasses a 2.5 km stretch of high-traffic highway, a bus terminal, and the nearby residential buildings. The numerical procedure is validated with some benchmark wind tunnel and numerical data. The locations of pollution accumulation have been identified and the effect of the ambient wind speed on the change of the maximum pollution concentration points has been investigated. Furthermore, potential pollution reduction strategies for such complex morphologies have been proposed based on the geometry change to prevent the formation of critical zones of pollution accumulation. The results show that critical zones are generally formed behind the walls of upstream buildings, adjacent to the upstream wall of the highway, and in front of the walls of downstream buildings. It was also found that changes in the ambient wind speed do not significantly alter the location of these critical zones. Furthermore, increasing the distance between adjacent buildings and the highway from 21 m to 30 m, can result in an average reduction of 76% in maximum carbon monoxide concentration values. An investigation into the impact of upstream buildings height indicates that reducing the height of certain buildings can effectively diminish pollution concentration to zero in residential areas and surrounding sidewalks. Additionally, increasing the depth of the highway and erecting 2 m solid barriers on either side of the highway are identified as two other effective techniques for reducing pollution concentration on both sides of the highway. The findings can be utilised to develop novel strategies aimed at enhancing air quality for residents and pedestrians.

The influence of the Biobio Canyon on the circulation and coastal upwelling/downwelling off central Chile

The influence of Biobio Canyon on the circulation and coastal upwelling and downwelling was evaluated by using high-resolution hydrodynamic simulations of the coastal ocean off central Chile, a highly productive zone of the Southern Humboldt Current System that supports important coastal ecosystem services. The model bathymetry was GEBCO 2020 with a spatial resolution of 500 m, which allows a relatively fine representation of the complex topography of the region. To understand the role of the Biobio Canyon, contrasting experiments were run. A Reference Simulation (RS) considering the canyon bathymetry and the Biobio River discharge, a Simulation Experiment 1 (SE1) considering only the canyon bathymetry without the Biobio River discharge, and a Simulation Experiment 2 (SE2) only with the Biobio River discharge (without the canyon). During upwelling conditions, the characteristic asymmetry in the along-canyon circulation was observed, with onshore flow over the northern side of the canyon and most of the upper water column, and offshore flow over the upstream canyon wall. In contrast, the cross-shore flow was limited to a narrow band over the northern wall of the canyon in RS and SE1 during downwelling conditions. Not many differences were detected between the RS and SE1 during both upwelling and downwelling conditions, which suggest that the canyon imposes a major effect on the circulation in comparison to the river discharge. Our results exhibit a net onshore transport onto the continental shelf during coastal upwelling and, net offshore transport restricted to the area south of the canyon during downwelling conditions. Finally, the advection of high-density waters on the northern shelf dominates during upwelling conditions in RS and SE1. Similarly, presence of the canyon promotes higher bottom density over the shelf even during downwelling conditions, as compared to the no-canyon experiment.

Experimental parametric study on two-phase spray cooling of HEF 7100 using flat spray nozzles

Spray cooling is an effective method and thus finds many applications, including thermal management and energy production. In this regard, spray cooling with flat sprays is particularly crucial in applications with space constraints, as it efficiently dissipates heat while conforming to limited spatial requirements. Flat sprays ensure a wide yet uniform distribution of coolant, making it ideal for cooling systems in compact and densely packed environments. Motivated by these advantages, in this study, we performed an experimental study on the effect of major flat spray parameters, which are the spray flow rate (160 mL/min), spray upstream pressure (2.5 and 5.0 bars), spray angle (0, 30, and 90°), spray distance (3 and 10 cm), spray temperature (23 and 61 °C) and wall heat flux (10–100 W/cm2), on the two-phase heat transfer performance. According to the results, unlike full cone nozzles, the spray distance enhances heat transfer. While the cooling rate increased with the upstream pressure, the inlet temperature had a deteriorating effect on the heat transfer performance of the system. The results also indicated that the heat transfer rate decreased with the spray angle. The spray angle-induced effect on the system performance diminished with the spray pressure and temperature. The obtained results were used to develop two correlations for single phase and nucleate boiling heat transfer in flat sprays. The findings of this study suggest that flat sprays, with their significantly lower flow rates compared to full spray cone nozzles, effectively meet the requirements for direct thermal management by offering precise control, reduced fluid consumption, and enhanced efficiency.

Interaction Length Analysis of Hypersonic Shock–Boundary-Layer Interaction Including High-Enthalpy Effects

Shock-induced flow separation plays an important role in practical applications of hypersonic flows, like intake unstart and unsteady aeroelastic phenomena. The size of the shock–boundary-layer interaction (SBLI) region determines the extent of choking in engine inlet ducts. Recent scaling studies have shown that interaction length is a function of shock-induced pressure jump, upstream Mach number, Reynolds number, wall heating/cooling, and flow deflection angle at the shock wave. In this work, we analyze the interaction length scaling for different hypersonic SBLI cases at varying flow enthalpies. We perform Reynolds-averaged Navier–Stokes (RANS) simulations, using a shock-strength-dependent turbulence model, of hypersonic SBLI flows over a range of Mach numbers and at flow enthalpies where vibrational nonequilibrium starts to be important. The numerical results closely match the experimental data, validating the RANS predictions. The size of the separation bubble in such high-enthalpy cases is found to not follow the known scaling relations. We propose a modified scaling that collapses a wide range of hypersonic Mach numbers and freestream enthalpy conditions. We also derive the high-Mach-number limit to demonstrate that wall cooling/heating and geometry are the main factors affecting the size of the SBLI region in hypersonic flows.

Controlling Seepage Flow Beneath Hydraulic Structures: Effects of Floor Openings and Sheet Pile Wall Cracks

Using one opening (filter) within the floors of hydraulic structures is a known technique to relieve the seepage effects on their floors. In this study, a new method to control seepage flow by using two identical filters instead of one was tackled numerically. A comparative analysis of using one versus two filters was conducted for different thicknesses of the permeable stratum, apron size (b), filter length, and sheet pile wall depths. Results indicate that two filters are considerably more effective than using one where the overall uplift force, the maximum potential head, and the hydraulic exit gradient downstream of the floor are reduced to 42–56%, 42–51%, and 66–76%, respectively, compared to one filter, while slightly increasing seepage flow by 1–7%. Many reasons can lead to horizontal openings (cracks) appearing along the sheet pile walls beneath hydraulic structures. The current study tackled their effects on seepage flow for the first time and examined their impact on the floor. A crack in the upstream sheet pile wall can increase total uplift forces by up to 40%, while a crack in the downstream sheet pile wall can increase the hydraulic exit gradient by up to 230%

Optimal design of diversion dams based on Upgraded Multi-Objective Particle Swarm Optimization (UMOPSO)

In the present research, the application of the Upgraded Multi-Objective Particle Swarm Optimization (UMOPSO) algorithm is investigated with the aim of reducing concreting costs through minimizing the diversion dam weight function. At the time of using optimization algorithms, regarding the nature of such a;gorithms and the research problem, making changes in algorithm running steps is necessary. Hence, in the current study, making some changes in the UMOPSO algorithm develops it to solve multi-objective optimization problems by considering their problem restrictions. In this paper, the study diversion dam is the Nazelian dam located in Kermanshah, Province, Iran. The decision variables in the optimization problem include the height of upstream and downstream cut off walls, the length and the thickness of the upstream concrete bed and the thickness of the stilling basin. By considering the initial population (n = 15), the optimal answer is achieved in iteration 100 and after algorithm run 1500. In this study, considering that the objective function should be evaluated for five decision variables with infinite possible values on a discrete range for each variable, the general result of the present study emphasizes on the desirable efficiency and speed of the UMOPSO algorithm in finding the optimal answer of the diversion dam optimal design problem which shows the strength of this algorithm.

Associations between atherosclerotic luminal stenosis in the distal internal carotid artery and diffuse wall thickening in its upstream segment.

Significant atherosclerotic stenosis or occlusion in the distal internalcarotid artery(ICA) may induce diffuse wall thickening (DWT) in the upstream arterial wall. This study aimed to assess the association of atherosclerotic steno-occlusive diseases in the distal ICA with DWT in the upstream ipsilateral ICA. Individuals with atherosclerotic stenosis in the distal ICA, detected by carotid MR vessel wall imaging using 3D pre- and post-contrast T1 volume isotropic turbo spin-echo acquisition (T1-VISTA) sequence, were enrolled. The associations of vessel wall thickening, the longitudinal extent of DWT, enhancement of the upstream ipsilateral ICA, and stenosis degree in the distal ICA were examined. Totally 64 arteries in 55 patients with atherosclerotic steno-occlusive distal ICAs were included. Significant correlations were found between distal ICA stenosis and DWT in the petrous ICA (r = 0.422, p = 0.001), DWT severity (r = 0.474, p < 0.001), the longitudinal extent of DWT in the ICA (r = 0.671, p < 0.001), enhancement in the petrous ICA (r = 0.409, p = 0.001), and enhancement degree (r = 0.651, p < 0.001). In addition, high degree of enhancement was correlated with both increased wall thickness and increased prevalence of DWT in the petrous ICA (both p < 0.001). DWT of the petrous ICA is commonly detected in patients with atherosclerotic steno-occlusive disease in the distal ICA. The degree of stenosis in the distal ICA is associated with wall thickening and its longitudinal extent in the upstream segments. Diffuse wall thickening is a common secondary change in atherosclerotic steno-occlusive disease in the intracranial carotid. This phenomenon constitutes a confounding factor in the distinction between atherosclerosis and inflammatory vasculopathies, and could be reversed after alleviated atherosclerotic stenosis. • Diffuse wall thickening of the petrous internal carotid artery is commonly detected in patients with atherosclerotic steno-occlusive disease in the distal internal carotid artery. • The phenomenon of diffuse wall thickening could be reversed after stenosis alleviation. • Carotid artery atherosclerosis with diffuse wall thickening should warrant a differential diagnosis from other steno-occlusive diseases, including moyamoya diseases and Takayasu aortitis.

Systemic immune-inflammation index is associated with ulcerative plaque in patients with acute ischemic stroke: A single center exploratory study.

This study explored the correlation between inflammatory markers and ulcerative plaques based on carotid doppler ultrasound (CDU) in individuals with acute ischemic stroke (AIS). A total of 202 cases diagnosed with AIS associated with atherosclerotic plaque (AP) in the carotid artery were enrolled in this research. Collecting clinical baseline data, laboratory data (such as the complete blood count) and imaging data (CDU and Brain magnetic resonance imaging [MRI]). Then the correlation between Systemic immune-inflammation index (SII, SII = P N/L, where P, N, and L were the peripheral blood platelet, neutrophil and lymphocyte counts, respectively), the shape and position of AP, the degree of carotid artery stenosis, and the presence of ulcerative plaques. Cutoff values were determined accordingly. SII and high sensitivity CRP (hs-CRP) were independent risk factors for the presence of vulnerable carotid plaques. SII, type A plaque, plaque above carotid bifurcation, and severe carotid stenosis were independent risk factors for the presence of ulcerative plaque. The AUC value, the sensitivity, specificity, the best cutoff value of SII in predicting the presence of ulcerative plaque was 0.895, 93.3%, 89.2%, and 537.4 (109 /L), respectively. SII at admission was found to be independently associated with the presence of AIS with vulnerable plaque, especially ulcerative plaques. Moreover, plaque ulceration was more likely to form when the area of higher plaque thickness was located in the upstream arterial wall of maximum plaque thickness (WTmax), plaque was above the carotid bifurcation and severe carotid stenosis.

Significance of Upstream Wall Conditions in Characterizing the Heat Transfer Phenomena of Rarefied Flows

Abstract Slip flows in small-scale flow networks involve simultaneous presence of multiple factors governing the flow field. In addition, conditions of upstream wall need to be clearly defined for quantifying the total heat that fluid receives from the wall. The present work addresses these aspects by analyzing the heat transfer aspects of slip flow of gaseous nitrogen through a circular pipe, undergoing either heating or cooling. The complete form of the governing equations is solved numerically while retaining property variation. The thermal field is found to exhibit two distinct asymptotic regions, with the first one representing fully developed heat transfer and the second one representing isothermal states. The fully developed Nusselt number (Nufd) is found to rise first, before dropping continuously with rise in Knudsen number (Kn). The pair of Kn and maximum Nufd is found to be dependent on Peclet number (Pe) of the system. Local Nu is found to drop to a minimum, lower than Nufd for Kn∼O(10−3) due to a significant radial advection. The presence of an adiabatic upstream wall reveals that heat may propagate up to the inlet for Kn≳0.015. An analytical solution is developed to approximate this limiting value of Kn, and it agrees well with the numerical results. The observed flow behavior leads to the categorization of flow regime into three types: (i) Kn&amp;lt;0.001, possessing dependence on change in Pe only, (ii) 0.001≤Kn&amp;lt;0.01, possessing concurrence of effects due to change in Pe and Kn, and (iii) 0.01≤Kn&amp;lt;0.1, possessing dependence on change in Kn only. Further, Pe is shown to represent Nubulk for the flow, where in the range 0.01≤Kn&amp;lt;0.1, Nutot≈Nubulk as Kn approaches 0.01 and Nutot≈Nuin as Kn approaches 0.1. A convenient approach is proposed to evaluate Nutot for any condition of the upstream wall. These outcomes indicate the necessity to clearly define the condition of the upstream wall and to evaluate the total heat transfer in small-scale heat exchangers, which may be much larger than what fluid carries downstream.

Structural Condition Assessment and Temperature Effect Analysis of Cable–Pylon Anchorage Zone Using Long-Term SHM Data

To study the temperature effect in the anchorage zone of a long-span cable-stayed bridge, a wavelet multiresolution analysis and layered stripping method were used in this paper. Based on long-term structural health monitoring data, the signals were decomposed and reconstructed at multiple time scales, confirming temperature to be the main factor causing the stress change in the anchorage zone. The results of structural condition during operation showed that the general compressive stress level of the anchorage zone was low. However, the tensile stress level of the sidewall was high, which led to a severe concrete cracking. Excluding the influence of the seasonal temperature, the compressive stress increased slightly, the horizontal tensile stress in the upstream inner pylon wall increased, and the crack width increased gradually. By analysing the daily temperature effect on the upstream and downstream pylon walls, the regression model proposed in this study can be used to predict the daily temperature effect at any time in the diurnal cycle. The accuracy of the model is reliable within 6 days, but for the location of severe cracks, the monitoring data should be updated in real time to ensure the precision.

Acoustics in Flameholding Solid-Fuel-Ramjet Fuel Grains

Experiments are conducted to determine how acoustic perturbations affect the performance and flameholding of solid-fuel ramjets with nonstandard combustion chambers. The focus is on the effect of wall cavities carved in the fuel grain using additive manufacturing. An improved understanding of how the wall geometry contributes to the establishment of acoustic modes is sought. A novel combustion mechanism was developed using a counterflow burner to study the combustion and regression of solid model fuel polymethyl methacrylate. The diffusion flame between the fuel and oxidizer was studied numerically using a solid-fuel decomposition and melt layer model to simulate convection and pyrolysis of the material. This model was validated using new experimental data as well as previously published works. The foam layer parameters are critical to the success of the validation, showing that the increased residence time of the gas in the bubbles facilitates the fuel breakdown. Fourth-order computational simulations of ramjet combustion without regressing fuel walls using a novel discontinuous Galerkin approach are performed with a fully conjugate solution for the thermal wave in the solid. Turbulent transport strongly affects the heat feedback to the walls, and low-frequency vortical modes (e.g., with a vortical wavemaker) associated with a recirculation region at the injector upstream wall are linked to an increase in chamber pressure and fuel mass flux.

Spray characteristics of a liquid fuel jet in a tandem backward-facing step cavity in supersonic flow

The influences of the backward-facing step and injection schemes on the spray characteristics of a liquid jet in a tandem backward-facing step cavity were investigated experimentally using high-speed photography and Schlieren techniques under a Mach 2.0 supersonic crossflow for the first time. The instantaneous structures of the spray and wave structures of the air-flow field were accurately captured by the high temporal resolution experimental technology. It is found that exist complex wave structures in the tandem backward-facing step cavity. A dimensionless spray factor was defined to describe the concentration of spray inside the cavity qualitatively. As a result, it is revealed that for the short backward-facing step cavity, the injection scheme near the upstream inlet has a higher penetration depth but a lower spray distribution, while the injection scheme near the cavity has a more spray distribution. For the long backward-facing step cavity, the injection scheme near the upstream inlet also has a higher penetration depth and the injection scheme near the sharp corner of the backward-facing step has a more sufficient spray distribution. The long backward-facing step cavity-based combustor with the upstream wall transverse injection is an optimized injection scheme to both improve penetration and spray distribution inside the cavity. Finally, a penetration depth formula is proposed to explain the spray distribution behaviors in a tandem backward-facing step cavity.

A Comparative Study of Film Cooling with Combined Impingement and Film Cooling

In order to cool the gas turbine vane or blade and raise the operating temperature, two standard convective cooling methods are used: jet impingement cooling and film cooling. The current study uses computational analysis to analyze and compare film cooling effectiveness with and without multi-jet impingement cooling on a flat plate. Ansys Fluent software is used to perform computational analysis on a flat plate. The computational results were compared with the experimental result using K-ω SST turbulence model and validated with literature data. The flat plate is used for the conjugate heat transfer study on the hot surface (named interaction surface), cold surface (called target surface), and inside the film hole. Different heat transfer parameters such as heat flux, Nusselt's number, and effectiveness are compared for the two cases, i.e., film cooling with impinging jets (IFC) and film cooling without impingement cooling (FC). It is observed that the FC case shows lower effectiveness as compared to the IFC case. The average Nusselt number for the IFC cases is almost three times larger than FC. The film exit temperature values are higher FC case, but it is more uniform in the IFC case. Interaction surface heat flux and Nusselt number values show higher values on the upstream wall of the film hole for IFC than in the FC case.

Gliding Arc Plasma-Controlled Behaviors of Jet-Wake Stabilized Combustion in a Scramjet Combustor

A multichannel gliding arc (MCGA) plasma is used to control the jet-wake stabilized combustion in a Mach 2.92 cavity-based and ethylene-fueled scramjet combustor. Optical diagnostic methods (including high-speed photography, schlieren, and planar laser-induced fluorescence with acetone tracer) combined with plasma kinetic simulations and Reynolds-averaged Navier–Stokes/large-eddy simulations were employed to investigate the combustion behaviors. The results show that the flame is mainly located near the cavity leading edge and operated at the jet-wake stabilized mode. When the MCGA plasma is added at the upstream wall of the cavity leading edge, the vigorous region of the flame spreads upstream 6.5 times longer than the original one without the plasma. Once the plasma is turned off, the flame returns back to the cavity leading edge. The species (Nitrogen in the excited state) and hydrogen atoms produced by the plasma are favorable for igniting the flame near the plasma, and the average penetration depth of the fuel above the plasma is increased by about 10%. Intense combustion near the plasma occurs with higher pressure, establishing the recirculation zone with the boundary-layer separation. The combined effects of the reactive species produced, the elevated temperature, and the modified flowfield induced by the plasma contribute to the MCGA-controlled behaviors of jet-wake stabilized combustion.

Numerical investigation of upward supercritical water flow with Joule heating effect

By using Joule's Law, resistivity equation, Piecewise Cubic Hermite Interpolating Polynomial (PCHIP) functions, and resistivity-temperature correlation established based on the available experimental data, Joule heating heat flux profiles of upward supercritical water heat transfer in the annular tube are modelled and applied as the boundary condition. The calculated wall temperature result is compared to the Razumovskiy et al. experiment dataset. It is discovered that by using the PCHIP-modelled Joule heating profiles, the estimated wall temperature for both the Normal Heat Transfer (NHT) and Deteriorated Heat Transfer (DHT) regimes is comparable to the experiment in terms of trend and magnitude. However, by using the conventional constant heat flux profiles, the upstream and downstream wall temperatures are overestimated and underestimated respectively. This study demonstrated that CFD can adequately replicate experimental heat flux conditions, hence improving CFD estimates of supercritical heat transfer, particularly for the DHT, which has a substantial heat flux variation. With this fine imitation of the Joule heating heat flux, the critical DHT regime can be more accurately predicted.

Two distinct types of turbidity currents observed in the Manila Trench, South China Sea

Sediment gravity flows are the most direct and efficient transport mechanisms for moving terrestrial sediments into deep oceans. Scarcity of firsthand measurements, however, has hindered the quantitative, even qualitative characterization of such flows. Here we present a unique year-long data record from ~4000 m depth in the Manila Trench that captured two very different gravity flows in terms of their hydraulic and sedimentary properties. The first flow was of slow speed (~40 cm s−1) and long duration (~150 h), thus nicknamed ‘Tortoises’, and carried very fine sediment with low concentration (~0.01%). The fast (~150 cm s−1) but short-lived (~40 h) flow, nicknamed ‘Hares’, carried much coarser sediment with higher concentration (~1.2%). Clay mineral compositions suggest that the ‘Tortoises’ originated from upstream canyon wall slumping, whereas the ‘Hares’ was likely submarine canyons southwest of Taiwan Island due to typhoon. Grain size is a key factor in determining evolution of turbidity currents.

The linear-time-invariance notion of the Koopman analysis. Part 2. Dynamic Koopman modes, physics interpretations and phenomenological analysis of the prism wake

This serial work presents a linear-time-invariance (LTI) notion to the Koopman analysis, finding consistent and physically meaningful Koopman modes and addressing a long-standing problem of fluid mechanics: deterministically relating the fluid excitations and corresponding structure reactions. Part 1 (Li et al., Phys. Fluids, vol. 34, no. 12, p. 125136) developed the Koopman-LTI architecture and applied it to a pedagogical prism wake. By a systematic analytical procedure, the Koopman-LTI generated sampling-independent linear models that captured all the recurring dynamics embedded in the input data, finding six corresponding, orthogonal, and in-synch fluid–structure mechanisms. This Part 2 analyses the six modal duplets to underpin their physical implications, providing a phenomenological analysis of the subcritical prism wake. Visualizing the newly proposed dynamic Koopman modes, results show that two mechanisms at St1 = 0.1242 and St5 = 0.0497 describe shear layer dynamics, the associated Bérnard–Kármán shedding and turbulence production, which together overwhelm the upstream and crosswind walls by instigating a reattachment-type of reaction. The on-wind walls’ dynamical similarity renders them a spectrally unified fluid–structure interface. Another four harmonic counterparts, namely the subharmonic at St7 = 0.0683, the second harmonic at St3 = 0.2422, and two ultra-harmonics at St7 = 0.1739 and St13 = 0.1935, govern the downstream wall. Finally, this work discovered the vortex breathing phenomenon, describing the constant energy exchange in the wake's circulation-entrainment-deposition processes. With the Koopman-LTI, one may pinpoint the exact excitations responsible for a specific structure reaction, benefiting future investigations into fluid–structure interactions and nonlinear, stochastic systems.

Characteristics of penetration and distribution of a liquid jet in a divergent cavity-based combustor

The atomization process of a liquid jet in a divergent cavity-based combustor was investigated experimentally using high-speed photography and schlieren techniques under a Mach number 2.0 supersonic crossflow. Gas-liquid flow field was studied at different divergent angles and injection schemes. It is found that complex wave structures exist in the divergent cavity-based combustor. The spray field can be divided into three distinct zones: surface wave-dominated breakup zone, rapid atomization zone and cavity mixing zone. A dimensionless spray factor is defined to describe the concentration of spray inside the cavity qualitatively. As a result, it is revealed that for the large divergent angle cavity, the injection scheme near the upstream inlet has a higher penetration depth but a lower spray distribution, where the injection scheme near the cavity has a more spray distribution. For the small divergent angle cavity, the injection scheme near the upstream inlet also has a higher penetration depth and the injection scheme near the start point of the divergent section has a more sufficient spray distribution. The small divergent angle cavity-based combustor with the upstream wall transverse injection is an optimized injection scheme to improve both penetration and spray distribution inside the cavity. Finally, a penetration depth formula is proposed to explain the spray and distribution behaviors in the divergent cavity-based combustor.

Influence of upstream wall slope on aerodynamic properties of a rectangular cavity in rarefied hypersonic flows

A comprehensive numerical study is performed to reveal influence of the upstream wall slope on aerodynamic properties as rarefied hypersonic flows pass through the two- and three-dimensional cavity configurations using the direct simulation Monte Carlo (DSMC) method. In this work, slope effect is obtained by the cavity upstream wall sloping upward and downward as well as bulging partially. Besides, cases of freestream with different velocities and altitudes are taken into account to examine the slope effect. It is found that cavity flow is transformed from the open type to the transitional and finally to the closed as the cavity upstream wall gradually slopes upward, promoting the heat energy exchange between gas molecules with the cavity floor, and meanwhile achieving a considerable rate of decrease of the peak heat flux on the reattachment corner. However, the upstream wall sloping downward or bulging partially seems to only have a slight effect on cavity flow and wall heat flux. It's worth noting that slope effect is closely related to velocity and altitude of the freestream, such as the greater the velocity and altitude, the stronger the slope effect. For the three-dimensional cavity configuration, influence of the upstream wall slope on cavity flow and wall heat flux is weaken considerably due to the three-dimensional effect. From the perspective of optimal design for the hypersonic thermal protection system, the cavity with an upward upstream wall is proposed because it not only reduces peak heat flux on the reattachment corner, but also decreases gas temperature inside the cavity.

Gap ratio effects on the coherent structures surrounding a near-wall square cylinder

Turbulent flow around a square cylinder offset at different heights from an upstream rough wall are investigated using time-resolved particle image velocimetry. The Reynolds number based on the freestream velocity and cylinder height was 12750 and the incoming turbulent boundary layer thickness was 7.2 times the cylinder height. The examined gaps beneath the cylinder were 0.0, 0.3, 0.5, 1.0, 2.0, 4.0, and 8.0 times the cylinder height. The results show that the mean recirculation length remains relatively constant for cases with a gap size greater than or equal to 1.0 times the cylinder height. Flow acceleration through the gap under the cylinder is strongest with a gap size of 2.0 times cylinder height. Downstream of the cylinder, the vertical velocity fluctuations contribute strongly to the production of turbulent kinetic energy by interacting with mean wall-normal velocity gradient in the vertical direction. The dominant frequency embedded in the incoming turbulent boundary layer persists in the wake of the cylinder when the gap size is less than 1.0 cylinder height. Kelvin-Helmholtz vortices originating from the top leading edge of cylinder occur at Strouhal numbers of 2.77-3.11 for the cases with gap ratio ranging from 0.3-8.0 cylinder heights, while spectral migration towards lower frequencies is observed for cases with gap sizes equal to 0.3 and 0.5 cylinder heights. The spatial trajectory of top Kelvin-Helmholtz vortices extends further away from the cylinder as the gap size increases. Spectral proper orthogonal decomposition analysis shows that the von Kármán vortex shedding structures possess the highest spectral energy for all offset cases despite regular vortex shedding being disrupted when the gap size is 0.3 times the cylinder height.