Surface Blowing Research Articles

New scaling law for turbulent boundary layers with high surface mass transfer

A fundamental understanding of the phenomena occurring in the turbulent boundary layer in the presence of surface blowing is limited, and considerable disagreements persist even in describing primary quantities, such as the boundary layer profile. The theories based on the linear boundary layer equations show that the thickness of the sublayer increases in the presence of surface blowing; therefore, the viscous sublayer and law of the wall modify. In this study, direct numerical simulations (DNS) of turbulent boundary layers with uniform surface mass transfers are carried out in order to scale the velocity profile. Emphasis is placed on moderate to high mass transfer rates, which are relevant to the most common hybrid rockets configuration. DNS data are used to establish a functional law of the wall and a law of wake by means of the relation between the wall shear stress and surface mass transfer. Analysis of the mean kinetic energy budget shows that the magnitude of turbulent kinetic energy increases by surface mass transfer, and the production rate extends significantly in the inner layer as the injection rate increases. DNS data of various surface blowing are used to complete the closure of turbulence kinetic energy equation and develop an eddy viscosity model. The predicted turbulent kinetic energy and eddy viscosity agree with DNS data for moderate to high blowing rates.

Large-eddy simulation of a planar offset-jet with heat transfer: The effects of ventilation

Several industrial and engineering applications employ ventilated offset-jets such as flow separation control devices, upper surface blowing used in short take-off and landing aircraft, drying processes, and fuel injection systems. The design and efficient operation of these devices relies on controlling the turbulence levels, heat transfer rates, and the locations of flow reattachment points. Therefore, an understanding of the flow and thermal characteristics of ventilated offset-jets helps in the design and operation of these devices. The present study aims to address this by performing large-eddy simulations (LES) of a planar turbulent offset-jet for different velocities of the ventilated-jet. To understand and quantify the effects of ventilation on the offset-jet, a range of velocity-ratios of 0, 0.1, 0.14, and 0.18 are considered for a primary offset-jet Reynolds number of 14,000. First, a grid-sensitivity study is performed to establish the convergence of the solver and the LES index of quality of resolution is obtained to ascertain the quality of the mesh. Thereafter, the mean and second-order statistics of the streamwise component of velocity are compared with reference data from the literature to validate the numerical solver. The effects of ventilation on the offset-jet are studied using the decay of streamwise velocity, jet-spread, the evolution of pressure coefficient, friction coefficient, the mean Nusselt number, and the unsteady characteristics. As the velocity-ratio increases, the suction pressure in the initial region of the domain decreases, which causes the jet to spread away from the bottom wall. Further the peak magnitudes of the mean Nusselt number and coefficient of pressure decrease exponentially with an increase in the velocity-ratio. The peak magnitude of the mean Nusselt number for the unventilated-jet case – the offset-jet without the ventilated-jet – is the largest and its value for the ventilated-jet cases decreases as the velocity-ratio is increased. It is concluded that in the initial region, the ventilated-jet has a profound effect on the offset-jet flow and as the flow develops into a wall-jet the effects of ventilation vanish.

Evaporation characteristics of ethanol droplet in different gases under force convection condition

Ethanol is recognized as one of the finest alternative bio-fuels due to its natural characteristics. Fundamental studies on ethanol droplet evaporation process are mainly either in stagnant environment or using some semi-empirical co-relations. Here, a fully numerical model based on the first principle is solved to investigate the effects of various atmospheric gases (Ar, N2, O2 and CO2) on droplet evaporation phenomenon under force convective situation. Two-dimensional governing equations of species, momentum and energy transfer of spherical coordinate system are solved, and the simulation is validated quantitatively with the literature. Uniform convective strength (Re = 100) is maintained for all cases examined at T∞ = 500 K and P∞ = 0.1 MPa. From the simulation results, it is observed that the viscosity ratio (liquid to gas) has effect on droplet life time. The ethanol droplet life time increases in less viscous atmospheric gases. The ethanol droplet life time is shorter in Ar gas, but heat-up period, wet-bulb temperature, and the surface blowing effect are higher in Ar compared to other atmospheric gases. The heat-up periods of the ethanol droplet in Ar, CO2, N2 and O2 atmosphere occupy around 30%, 20%, 17% and 16.5% of the total life time of the droplet, respectively. It is also noticed that, the heat-up period increases with increase in the thermal conductivity ratio (liquid to gas) and vice versa. Furthermore, the flow pattern of both gas- and liquid-phase in terms of streamline and the internal temperature distributions of the droplet are visualized at various time instants.

Numerical Investigation of an Unsteady Blade Surface Blowing Method to Reduce Rotor Blade-Vortex Interaction Noise

This paper presents a numerical investigation of unsteady surface blowing using periodic variations of jet velocity with azimuthal angle to reduce helicopter rotor blade-vortex interaction (BVI) noise. The unsteady blowing is modeled as the mass flow outlet boundary condition of time-varying jet velocity on the blade surface grid using computational fluid dynamics. The same high-resolution overset grid system and flow/noise solver are used to perform a detailed flow field simulation and noise prediction for the nonblowing baseline case and the steady/unsteady blowing cases under the rotor BVI condition, and a grid convergence study for the steady and unsteady blowing cases is carried out. The BVI noise reduction and rotor thrust coefficient results of the unsteady blowing method and the previously published steady blowing with constant jet velocity are then compared. The noise reduction level of unsteady blowing is approximately equivalent to that of steady blowing (noise reduction is more than 3 dB). However, the loss in rotor thrust coefficient caused by unsteady blowing (3.3%) is only half of that by steady blowing (6.3%); the air mass cost by unsteady blowing is only 63.7% of that by steady blowing per rotation revolution. The results show that unsteady blowing can effectively reduce BVI noise with lower cost and less thrust loss.

Parametric Effect of Blade Surface Blowing on the Reduction of Rotor Blade–Vortex Interaction Noise

AbstractReducing rotor blade–vortex interaction (BVI) noise is a major challenge in rotorcraft research. This paper presents a high-resolution numerical study of the effects of blade surface blowin...

Interaction between surface blowing and re-entrant jet in active control of hydrofoil cavitation

Numerically simulations were performed to study the effect of active flow control (through steady blowing) on unsteady cavitation over a modified scaled-down Francis turbine (hydrofoil). Large eddy simulations (LES) using a one-equation subgrid-scale model and the Sauer & Schnerr cavitation model, all available in OpenFOAM, served as the main numerical tool for the study. An angle of attack of 9° and a cavitation number σ=1.56 were chosen for study. The dynamics of the re-entrant jet and its role in the shedding of the cavitation sheet were elucidated. For active flow control, wall jet injection through a slot on the top surface of hydrofoil was considered for two different injection velocities of Uinj=0.88U0 and Uinj=1.42U0, where U0 is the inflow velocity. Quantitative comparisons to previously published measured data were used for model validation. It was found that the interaction of the re-entrant jet with the wall jet causes large fluctuations in pressure, thereby counteracting the local rise in pressure due to the wall jet. Thus, the effectiveness of wall jet injection to mitigate cavitation is limited under the operating conditions studied here.

Numerical Modeling and Parametric Study of the Melting Behavior of Ice Crystal Particles

Ice crystal icing has been identified as a risk to flight safety, due to its ability to reduce engine performance and its potential to cause engine damage and flameout. A critical factor in the sticking efficiency of ice crystals is the melting behavior of ice crystals. This paper presents an ice melting model integrated with surface blowing and porosity. A parametric study is performed to understand the effects of flow conditions (total pressure, humidity, total temperature, Mach number, and slip velocity) and particle properties (particle size, aspect ratio, and porosity factor) on ice melting behavior. The model is compared against the melting time of single ice particles in an acoustic levitator. The results show that employing sphericity and porosity could improve prediction performance. Surface blowing from evaporation increases melting time, and its effect is larger with a higher gas temperature. Ice melting time increases with pressure at high humidity, whereas an opposite trend is observed at low humidity. Increasing gas temperature, humidity, slip velocity, aspect ratio, and particle temperature can decrease ice melting time, whereas increasing Mach number, particle size, and porosity factor can increase ice melting time. Although wet bulb temperature can be used to evaluate ice melting potential when only one flow parameter is varied, it appears insufficient alone under scenarios where multiple flow parameters are changing simultaneously.

Numerical investigation on noise reduction of rotor blade-vortex interaction using blade surface jet blowing

The effects of blade surface jet blowing on the reduction of rotor blade-vortex interaction (BVI) noise are investigated using the compressible Reynolds-averaged Navier-Stokes equations and the Ffowcs Williams-Hawkings equations. A detailed analysis of the accuracy of the Weighted Essentially Non-Oscillatory (WENO) scheme and Monotone Upstream-Centered Scheme for Conservation Laws for simulating the flow field and predicting the BVI noise is performed using a two-bladed scaled AH-1 helicopter main rotor undergoing BVI as the baseline case. Additionally, a grid convergence study is conducted. The BVI noise is accurately predicted using the fifth-order WENO scheme and high-resolution grid system (the prediction errors of the peak sound pressure are less than 4%). Surface boundary conditions are introduced to model the effect of jet blowing on the blade surface on the flow control of the baseline case. Blade surface blowing is analyzed near the trailing edge and the leading edge near the rotor tip. The results show that blowing near the blade trailing edge is effective for BVI mitigation by altering the tip vortex strength and increasing the blade-vortex miss distance. The sound pressure level is reduced by 2.6 dB, and the peak amplitude is attenuated by more than 30%, with a 9.8% loss in rotor thrust. Therefore, blowing near the trailing edge may be considered a promising approach for active BVI noise control.

Effects of nozzle geometry and active blowing on lift enhancement for upper surface blowing configuration

Upper Surface Blowing (USB) has been regarded as a promising lift enhancement technique for short-take-off and landing (STOL) applications. The present study conducted numerical investigations to further explore the potential capability of USB technique by passive and active flow control strategies. Computations were accomplished by a Reynolds-averaged Navier-Stokes solver enclosed by the two-equation k-ω shear stress transport turbulence model. The propulsion effect of the up-mounted engine was represented by the Turbine Powered Simulator (TPS). Results show that aerodynamic lift of using USB system is significantly enhanced due to the attenuation of plume edge roll-up vortex which is beneficial to the Coanda effect. Varying nozzle geometries can augment lift by alleviating the three-dimensional edge effect and thus delaying the rollup of plume with a thinner jet sheet. Compared to the passive flow control method by nozzle geometry modification, active constant blowing flow control technique shows a more promising potential for lift enhancement for USB powered system. Within the test cases, a 50% lift increment can be achieved by using constant blowing on the USB system.

Geometric Effects on Charring Ablator: Modeling the Full-Scale Stardust Heat Shield

The multidimensional geometric effects within the heat shield of the Stardust sample return capsule are studied over its reentry trajectory, using a material response solver. The solver models material charring, conductive heat transfer, surface energy balance, pyrolysis gas transport, and orthotropic material properties in three dimensions. The material response of the heat shield is computed using isotropic and orthotropic models for conductivity and permeability, and both results are compared to the 1-D model. The orthotropic and isotropic models predict dissimilar responses, especially in regard to the surface blowing rates and pyrolysis gas transport. Although both models yield similar results for the surface temperature near the stagnation region, the temperature distribution within the heat shield differs. The 1-D model appears to greatly underestimate the thermal response deeper in the material, especially late in the trajectory.

Numerical simulation of heat and mass transient behavior of single hexadecane droplet under forced convective conditions

A droplet evaporation model based on the direct numerical method and the high-fidelity interface capturing methodology is developed to simulate the evaporation process of liquid droplets under a high-temperature convective environment. The coupled Level-set and Volume of Fluid method is applied to describe the droplet interface. The phase change rate is derived directly from the mass fraction gradient at the droplet interface. After verifying the accuracy of our model with the experimental results and the analytical solution, the evaporation process of droplet under high temperature and forced convective condition is examined. The results revealed that convection breaks the uniform distribution of temperature and concentration. The temporal behaviors of Nusselt and Sherwood number under convective environment are similar, as classified by two stages, i.e. the transient variation period and steady-state stage. In addition, increasing ambient temperature from 500 K to 800 K promotes the reduction rate of droplet volume by around 5 times. However, it restrains the convective heat and mass transfer rates as a result of the reduction in Reynolds number and the intensity of surface blowing. It is deduced that high temperature is beneficial for conductive heat transfer but unfavorable to the convective heat and mass transportation. By contrast, enhancing free-stream velocity is exceedingly beneficial to the convective heat and mass transfer rates as the convective heat and mass transfer rates in steady-state are promoted by 25% and 27%, respectively, when the free-stream velocity grows from 0.5 to 2.0 m/s. Finally, the variation of Sh and Nu with the Reynolds number obtained in this work is compared with the results of empirical correlations proposed by Ranz-Marshall [1] and Clift et al. [2]. It is concluded that, in the present research range of 1.5≤Re≤12.8 and 500 K≤T∞≤800 K, the modified Ranz-Marshall correlation has a better capability in predicting the Sh and Nu. This reveals that when droplets expose to a high-temperature free-stream with a low Reynolds number, the dynamical effects induced by evaporation is active and cannot be neglected.

Inverse analysis of the time-dependent heat flux in stagnation point flow of incompressible fluid impinging on a cylinder with uniform surface suction-blowing using Levenberg–Marquardt method

ABSTRACT In this paper for the first time, a numerical code based on Levenberg–Marquardt method is presented for solving inverse heat transfer problem of axisymmetric stagnation flow impinging on a cylinder with uniform transpiration and to estimate the time-dependent heat flux using temperature distribution at a point. The effect of noisy data on the final result is studied. The maximum value of the sensitivity coefficient is related to estimation of exponential heat flux and its value is 0.1718 also the minimum value of the sensitivity coefficient is 7.50 × 10−6 which is related to the triangular heat flux. The results show that the parameter estimation error in calculating the triangular and trapezoidal heat flux is greater than the exponential and sinus–cosines heat flux because the maximum value of RMS error is obtained for these two cases, which are 0.458 and 0.492, respectively the reason for the increase in error in estimating these functions is the existence of points where the first derivative of the function does not exist. This method also exhibits considerable stability for noisy input data. In most cases, surface blowing decreases the prediction accuracy by removing the boundary layers from the surface whereas suction acts vice versa.

Radiation heat transfer in ablating boundary layer combustion theory used for hybrid rocket motor analysis

Marxman theory is often used for developing correlations of fuel regression rate for hybrid rocket motor analysis. Effects of radiation are accounted for in this theory as a perturbation to the non-radiating blowing limit allowing for the leading order effects of blowing blockage from radiation heat transfer to influence convective heat flux. The theory does not, however, account for the non-linear changes in radiative gas absorption properties to allow for tightly coupled descriptions of heat transfer and surface blowing. In this study, Marxman theory is expanded in a new fully coupled approach, employing Schvab-Zeldovich coupling functions, unsteady heat transfer response of the fuel, and solution of the gas-phase radiation heat transfer. To develop this theory, Marxman’s theory is generalized to allow for expanded functional forms of friction coefficient and account for changes in gas properties. To validate the modeling approach, measurements from a simplified slab burner experiment are conducted. Paraffin wax is used as the fuel and relatively low oxidizer fluxes are employed so the dominate effects of radiation heat transfer can be understood. Measurements of temperature, soot volume fraction, and fuel radiative heat flux rely on two-color pyrometry analyses using high-speed camera color images. Comparisons of model predictions to data indicate the new tightly coupled approach provides good predictions of regression rate, temperature, and radiative heat flux to the fuel surface over the range of oxidizer flow rates considered. Model sensitivity studies reveal commonly used one-way coupling strategies may result in significant over prediction in fuel regression rate that are most likely compensated for by errors in simplified treatments of radiation heat transfer.

Numerical Simulation of the Interaction of Unsteady Surface Blowing (Suction) with Turbulent Near-Wall Flow

The dynamic and thermal characteristics of unsteady near-wall flows are investigated numerically on the basis of two-parameter turbulence models under the conditions of high-turbulence free stream and the impact of disturbing blowing (suction) factors in the boundary layer through a permeable surface section. The mutual effect of time harmonic oscillations of the external inviscid stream velocity and the blowing flow rate density on the wall on the development of time-dependent heatand mass-transfer processes in turbulent flow is analyzed. The laws of variation in the flow and heattransfer properties are established when the flow rate density is given to be variable in time and constant. The basic mechanisms of the impact of blowing and suction on the permeable section and downstream are studied by comparing the calculated results.

Innovative Means of Surface Blowing towards Heat Alleviation for Hypersonic Flows

A computational-based investigation has been carried out to examine the heat transfer effects of normal and tangential blowing on blunt surfaces exposed to high Mach number flows. Experimental results from such studies where a controlled outflow is imposed upon a surface under hypersonic conditions are few and far between. Simple 3D axisymmetric configurations supporting flat front surfaces suited to accommodate uniform frontal flow or round-nosed conical bodies with a stepped shoulder to provide uniform parallel flow to the conical surface were selected from a NASA experiment for validation purposes. Uniform outflow of 0.55 to 1.0 kg/s was applied in the presence of the prevailing free stream of M=6.8 to assess the extent of heat transfer rate alleviation. The numerical simulation confirmed the experiment that the application of outflow in both cases leads to the surface heat transfer relief.

Effect of surface blowing on aerodynamic characteristics of tubercled straight wing

Recent research proves that wings with leading-edge tubercles have the ability to perform efficiently in post-stall region over the conventional straight wing. Moreover, the conventional straight wing outperforms the tubercled wing at a pre-stall region which is quintessential. Even though tubercled wing offers great performance enhancement, because of the complexity of the flow, the trough region of the tubercled wing is more prone to flow separation. Henceforth, the present paper aims at surface blowing – an active flow control technique over the tubercled wing to enhance the aerodynamic efficiency by positively influencing its lift characteristics without causing any additional drag penalty. Flow parameters like blowing velocity ratios and the location of blowing were chosen to find the optimised configuration keeping the amplitude and frequency of the leading-edge tubercles constant as 0.12c and 0.25c respectively. Numerical investigations were carried out over the baseline tubercled wing and tubercled wing with surface blowing at various blowing jet velocity ratios 0.5, 1 and 2 over four different chordwise locations ranging from 0.3c to 0.8c. The results confirm that blowing at various x/c with different blowing velocity ratios performs better than the conventional tubercled wing. Comparatively, blowing velocity ratio 2 at 0.3c shows peak performance of about 28% enhancement in the lift characteristics relative to the baseline model. Particularly, in the pre-stall region, 25–50% increase in aerodynamic efficiency is evident over the tubercled wing with surface blowing compared with the baseline case. Additionally, attempts were made to delineate the physical significance of the flow separation mechanism due to blowing by visualizing the streamline pattern.

Scale Interactions in Turbulence for Mountain Blowing Snow

Abstract Blowing snow particle transport responds to wind motions across many length and time scales. This coupling is nonlinear by nature and complicated in atmospheric flows where eddies of many sizes are superimposed. In mountainous terrain, wind flow descriptions are further complicated by topographically influenced or enhanced flows. To improve the current understanding and modeling of blowing snow transport in complex terrain, statistically significant timing and frequencies of wind–snow coupling were identified in high-frequency observations of surface blowing snow and near-surface turbulence from a mountain field site in the Canadian Rockies. Investigation of the mechanisms influencing near-surface, high-frequency turbulence and snow concentration fluctuations provided strong evidence for amplitude modulation from large-scale motions. The large-scale atmospheric motions modulating near-surface turbulence and snow transport were then compared to specific quadrant analysis structures recently identified as relevant for outdoor blowing snow transport. The results suggest that large atmospheric structures modulate the amplitude of high-frequency turbulence and modify turbulence statistics typically used to model blowing snow. Additionally, blowing snow was preferentially redistributed under the footprint of these same sweep motions, with both low- and high-frequency coherence increasing in their presence.

A suppression method for thermomechanical stresses during the laser processing of structural elements

A method to reduce thermomechanical stresses during the laser processing of a beam-like specimen along its side surface and a thin disk along its central circular hole is considered. Some analytical temperature expressions are obtained in both these cases. The problems of determining thermal stresses during the heating of specimens with consideration of heat transfer on their surfaces are solved. The solutions obtained with heat transfer and without it are compared. It is shown that the side surface blowing can be used to suppress the fracture of heated specimens.

Research, Development and Recent Patents on Aerodynamic Surface Circulation Control - A Critical Review

The ever increasing demand for better performance of aircraft aerodynamics, from the fundamental understand- ing of flight, requires enhanced circulation. Circulation Control and hence its enhancement can be achieved by surface blowing in the form of Coandjet and have always been referred to in the consideration of various flow control methods to enhance aerodynamic performance, along with continuous, synthetic and pulsed jets, compliant surface, vortex-cell, and the like. Coandjet has also been applied in the development of novel aircrafts for short takeoff, while another circu- lation enhancement technique known as Trapped Vortex Cavity (TVC) is currently being given significant considerations. It is with such motivation that salient features, progress and development of various techniques in circulation control are here identified, including some of the recent inventions that have been registered as patents in this area. The present work reviews the influence, effectiveness and configuration of airfoil surface blowing of Circulation Enhancement and Control of aerodynamic surfaces. The crux of the TVC active research is their stabilization, while Coandenhanced lift enhance- ment technique has, to a certain extent reached a stage that it can be easily implemented with advantage.

Progress and Development of Coandă Jet and Vortex Cell for Aerodynamic Surface Circulation Control – An Overview

Coanda jet has always been referred to in the consideration of various flow control methods to enhance aerodynamic performance, i.e. to enhance lift, reduce drag and delay stall at higher angle of attack, along with continuous, synthetic and pulsed jets, compliant surface, vortex-cell, and the like. Coanda jet has also been applied in the development of novel aircrafts for short take off, while another circulation enhancement technique known as Trapped Vortex Cavity (TVC) is currently being given significant considerations. It is with such motivation that within the aerodynamics surface blowing techniques, Coanda jet and vortex-cell will be reviewed, to assess their characteristics in dramatically alter the behavior of aerodynamic components such as airfoils, wings, and bodies. Capitalizing on a host of research and technology development efforts on Coanda and surface blowing circulation enhancement, the present work reviews the influence, effectiveness and configuration of airfoil surface blowing of Coanda-jet and Trapped Vortex Cavity in circulation enhancement and control of aerodynamic surfaces. The crux of the TVC active research is their stabilization, while Coanda enhanced lift enhancement technique has, to a certain extent reached a stage that it can be easily implemented with advantage.