Asymmetric Channel Flows Research Articles

A wall stress model to predict high-Reynolds number flow over rough walls

Wall-modeled large eddy simulation of turbulent channel flow with rough walls was performed for Reynolds numbers of Reτr=615–2080. The eddy viscosity parameter in a wall stress model was modified to include the effects of wall roughness. Symmetric and asymmetric channel flows were studied for transitionally and fully rough states with ks+=62 and 112, respectively. The wall shear stress was estimated based on the velocity field within the wall layer using the total shear stress term. For the wall layer, the matching point was located well above the height of the virtual roughness elements. The numerical predictions for the mean velocity profile and Reynolds stresses compared well with the experimental data. The prediction for the mean velocity shift due to roughness agreed with the experimental measurements and empirical correlations. For the asymmetric channel flows with roughness on only one wall, the numerical simulations correctly reproduced the displacement of the mean flow away from the rough wall. Overall, the proposed wall-model allows for the prediction of the effects of wall roughness without resolving the geometry of the roughness elements on the wall. This is consistent with the goal of performing large eddy simulations of wall-bounded flows at high-Reynolds numbers with minimal resolution of the wall region, which results in a much reduced computational cost.

Performance analysis of a double-pass solar air heater system with asymmetric channel flow passages

The present work seeks to address the forced convection heat transfer behaviour of a double-pass solar air heater system (DPSAHS) provided with asymmetric channel flow configuration used for solar drying of agro-products. Outdoor experiments were performed on a DPSAHS having a constant channel depth ratio of 1.5. Thermal response of the DPSAHS under different influencing parameters such as flow rate, channel depth, and thermophysical properties of the working fluid was experimentally determined. The influence of ambient parameters such as solar intensity, ambient temperature, wind speed, and relative humidity on the thermodynamic behaviour of the DPSAHS was also investigated. Among which, solar intensity and ambient temperature were found to be the major parameters influencing the energy and exergy efficiency followed by wind speed. Relative humidity was found to have the least percentage contribution towards the thermal characteristics of the system. Overall thermal efficiency and exergy efficiency were found to vary in the range of 20–41% and 5.6–18% at two different mass flow rates of 0.02 kg s−1 and 0.03 kg s−1, respectively. The results also inferred that the influence of thermophysical property variation on the thermodynamic performance depends upon the operating temperature range and on the nature of working fluid. Air temperature in the lower channel was found to be an average 3 °C higher than that of upper channel passage corresponding to two different mass flow rates. Hence, the thermodynamic behaviour of DPSAHS was found to be strongly influenced by the variation in channel depth, ambient parameters, and mass flow rate. The obtained experimental results were also compared with the available literatures.

Relative Effects of Asymmetry and Wall Slip on the Stability of Plane Channel Flow

The effect of wall velocity slip on the stability of a pressure-driven two-dimensional asymmetric channel flow is examined by considering Navier slip condition on the channel walls. The two-parameter families of mean velocity profiles are considered to approximate the underlying asymmetric basic flow. Competing effects of skewness and maximum velocity on the stability of the flow are explored for a range of model parameters. The Orr–Sommerfeld system of the asymmetric flow is solved using a Chebyshev spectral collocation method for both symmetric and non-symmetric type slip boundary conditions. Numerical results indicate that moderate asymmetry in the basic flow has a significant role on the stability of the Poiseuille-kind parallel/nearly parallel flows. Wall slip shows a passive control on the instability of the asymmetric flow by increasing or decreasing the critical Reynolds number and the set of unstable wave numbers. The stabilizing/destabilizing effect of slip velocity on the flow instability is weak or strong depending on the presence of velocity slip at the upper or lower wall. Velocity slip has a profound grip on the flow behaviour by changing the shear rate inside the perturbed flow.

Deformation of an elastic capsule in a rectangular microfluidic channel

In the present study we investigate computationally the deformation of an elastic capsule in a rectangular microfluidic channel and compare it with that of a droplet. In contrast to the bullet or parachute shape in a square or cylindrical channel where the capsule extends along the flow direction, in a rectangular channel the capsule extends mainly along the less-confined lateral direction of the channel cross-section (i.e. the channel width), obtaining a pebble-like shape. The different shape evolution in these two types of solid channels results from the different tension development on the capsule membrane required for interfacial stability. Furthermore, in asymmetric channel flows, capsules show a different deformation compared to droplets with constant surface tension (which extend mainly along the flow direction) and to vesicles which extend along the more-confined channel height. Therefore, our study highlights the different stability dynamics associated with these three types of interfaces. Our findings suggest that the erythrocyte deformation in asymmetric vessels (which is similar to that of capsules) results from the erythrocyte's inner spectrin skeleton rather than from its outer lipid bilayer.

20705 側壁に設けたピストンの往復運動により生じる非対称流路内流れのPIV計測(OS5-3【流れの計測と制御(3)】)

20705 側壁に設けたピストンの往復運動により生じる非対称流路内流れのPIV計測(OS5-3【流れの計測と制御(3)】)

106 流動変動を与えたバルブレス非対称流路の流れ場の計測(G01-7 マイクロ流れ,G01 熱流体部門)

106 流動変動を与えたバルブレス非対称流路の流れ場の計測(G01-7 マイクロ流れ,G01 熱流体部門)

Orthogonal wavelet analysis of counter gradient transport phenomena in turbulent asymmetric channel flow

In this paper four families of orthogonal wavelets are applied to analyze the turbulent counter gradient transport phenomena in fully developed asymmetric channel flows. The results show that: (1) In the instance of counter gradient transport, the principal scale of the coherent structure is responsible for the strong local counter gradient transport; (2) Counter gradient transport phenomena have a strong effect on the intermittency of turbulence; (3) Non-Gaussian part of the principal coherent structure is essential for counter gradient transport phenomena.

Scaling exponents in fully developed asymmetric channel flow

Wavelet analysis is applied to study the global and local scaling exponents in fully developed asymmetric channel flow. Global exponents are calculated by orthogonal wavelets and Extended Scaling Similarity (ESS). The results show that the flow in an asymmetric channel flow exhibits different characteristics of intermittency from that in a symmetric flow. It is also shown that the intermittency property of the streamwise fluctuations is different from that of vertical fluctuations, and the intermittency does not decay with the increase of the distance from the wall. In addition, the Continuous Wavelet Transform (CWT) method is found to be unreliable to calculate the local scaling components. Finally, it is pointed out that the existence and the significance of negative local scaling components need further study.

Modified Eddy Viscosity Model in Fully Developed Asymmetric Channel Flows

This study presents a modified eddy viscosity model for describing vertical distribution of turbulent eddy viscosity in one‐dimensional fully developed asymmetric channel flows, which cannot be obtained by the traditional eddy viscosity model. A two‐power law expression, used to describe vertical distribution of flow velocity, is shown to account quite well for the major characteristics of flow in channels of asymmetric roughness. The modified eddy viscosity model, utilizing the two‐power law expression, enables formulation of a continuous distribution of eddy viscosity with positive magnitudes of turbulent eddy viscosity for asymmetric channel flows. Examples are given showing that the distribution of eddy viscosity obtained by using the modified eddy viscosity model is comparable with that obtained using Launder and Spalding's k‐ε turbulence model. This modified eddy viscosity model can, therefore, provide a relatively simple means for estimation of turbulent eddy viscosity in asymmetric channel flows.

A Note on a Generalized Eddy-Viscosity Hypothesis

The standard eddy-viscosity concept postulates that zero velocity gradient is accompanied by zero shear stress. This is not true for many boundary layer flows: wall jets, asymmetric channel flows, countercurrent flows, for example. The generalized eddy-viscosity hypothesis presented in this paper, relaxes this limitation by recognizing the influence of gradients in the turbulent length scale and the shear. With this new eddy-viscosity concept, implemented into the standard k–ε model, predictions of some boundary layer flows are made. The modelling results agree well with measurements, where predictions with the standard eddy-viscosity concept are known to fail.

FLOW THROUGH CONSTRICTED OR DILATED PIPES AND CHANNELS: PART 2

Indented tube flows are discussed when the characteristic Reynolds number K is large, the oncoming flow is fully developed and the indentations are slowly varying. The range of the theory in an allied paper (1) is extended to steady or unsteady asymmetric channel flows with α = O(K–1), and to symmetric tube flows with α = O(K–1), where α is a typical slope of the constriction. In the first regime here an upstream interaction takes place over a large length scale, forced by the transverse pressure gradient acting in the channel core, although oscillations of sufficiently high frequency in the wall shape can suppress this upstream response. The long-scale interaction is believed to occur ahead of any severe asymmetric distortion in steady channel flow, however. The second regime involves an elliptic core flow as the indentation length is shortened, but the wall layer is effectively unaltered ahead of the indentation.

Linear Instability of Asymmetric Flow in Channels

A study of the linear stability of asymmetric channel flows is presented. Three one-parameter families of basic velocity which possess, respectively, no, one, and two inflection points are treated. The competing effects of stabilizing asymmetry and destabilizing vorticity distributions are discussed. An inviscid wave speed theorem which extends a result of Stuart to flows with two inflection points is proved.