Skin-friction, roughness functions and predictive correlations are presented for random roughness that has a Gaussian power spectral density distribution of surface elevations. The root-mean-square ( rms ) roughness height and the skewness of the probability density function are parametrically varied to investigate the role of these parameters in generating the friction at the wall. Results are presented for all roughness regimes, from hydraulically-smooth to fully-rough. Negative skewness (pits) had a much smaller influence on drag than positive skewness (peaks). Predictive engineering correlations for the equivalent sandgrain roughness height indicate that the rms roughess height and skewness are important scaling parameters. However, the scaling does not appear to be universal as different correlations are needed for surface roughness with positive, negative and zero skewness. Most surfaces collapse to a single roughness function in the transitionally-rough regime similar to the one developed by Nikuradase (1933) for uniform sand-grain roughness. The exceptions are the wavy surface (low effective slope) and the surface with high positive skewness.

Skin friction measurements of systematically-varied roughness: Probing the role of roughness amplitude and skewness

Staggered oriented airfoil shaped pin-fin heat sink: Investigating the efficacy of novel water based ferric oxide-silica hybrid nanofluid

A roughness scaling behaviour is tested by performing the direct numerical simulation (DNS) of a turbulent channel flow over three-dimensional sinusoidal rough walls. By systematically varying the roughness height .

Scaling of rough-wall turbulence by the roughness height and steepness

Direct numerical simulation (DNS) is performed for two wall-bounded flow configurations: laminar Couette flow at is shown to depend on a competing effect between slip velocity and turbulent losses due to the Reynolds shear stress contribution. Presence of trapped air in the cavities significantly alters near-wall flow physics where we examine near-wall structures and propose a physical mechanism for their behaviour. The fully wetted roughness increases the peak value of turbulent intensities, whereas the presence of the interface suppresses them. The pressure fluctuations have competing contributions between turbulent pressure fluctuations and stagnation due to asperities, the near-wall structure is altered and breaks down with increasing slip. Overall, there exists a competing effect between the interface and the asperities, the interface suppresses turbulence whereas the asperities enhance them. The present work demonstrates DNS over a realistic multiphase SHS for the first time, to the best of our knowledge.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112019004191

Wall-bounded flow over a realistically rough superhydrophobic surface

Direct numerical simulations (DNS) are performed for two wall-bounded flow configurations: laminar Couette flow at $Re=740$ and turbulent channel flow at $Re_{\tau}=180$, where $\tau$ is the shear stress at the wall. The top wall is smooth and the bottom wall is a realistically rough superhydrophobic surface (SHS), generated from a three-dimensional surface profile measurement. The air-water interface, which is assumed to be flat, is simulated using the volume-of-fluid (VOF) approach. The laminar Couette flow is studied with varying interface heights $h$ to understand the effect on slip and drag reduction ($DR$). The presence of the surface roughness is felt up to $40\%$ of the channel height in the wall-normal direction. A nonlinear dependence of $DR$ on $h$ is observed with three distinct regions. The DNS results are used to obtain a nonlinear curve fit for gas fraction $\phi_g$ as a function of $h$, where $\phi_g$ determines the amount of slip area exposed to the flow. A power law linear regression fit is used to obtain $b_{eff}$ as a function of $\phi_g$. For the turbulent channel flow, statistics of the flow field are compared to that of a smooth wall to understand the effects of roughness and $h$. Two interface heights, $h=h_{rms}$ and $h=h_{max}$ are simulated to study their effect on the behaviour of the flow, where $h_{rms}$ and $h_{max}$ are the rms and maximum peak of the roughness heights respectively. Results show that the presence of trapped air in the cavities significantly alters near wall flow physics. The fully wetted roughness increases the peak value in turbulent intensities, whereas the presence of the interface suppresses them. Overall, there exists a competing effect between the interface and the asperities, where the interface suppresses turbulence whereas the asperities enhance them.

The invention discloses a dynamic pressure blade type probe. The interstage measurement space of a miniaturized multistage axial-flow compressor is quite small, as the load of an engine is increased constantly, the unsteady flow feature inside the compressor becomes more and more salient, and this causes great difficulty to interstage flow field measurement of the compressor. On one hand, since a probe is required to have quite high frequency response, the diameter of the probe cannot be too small; and on the other hand, since the diameter of the compressor is quite small, the dimension of the probe is required not to be too large in order to prevent quite large obstruction. A conventional common measurement means cannot satisfy the test requirement. According to the invention, through directly fixing a miniaturized dynamic pressure sensor on the surface of a blade, the novel blade type probe for measuring dynamic pressure is formed. When measurement is performed through the blade type probe, the effect on a detected flow field is quite small, and accurate dynamic measurement data can also be obtained.

Dynamic pressure blade type probe

Direct numerical simulation of flow in a turbulent channel with a random rough bottom wall is performed at friction Reynolds number . The rough surface corresponds to the experiments of Flack et al. (Flow Turbul. Combust., vol. 104, 2020, pp. 317–329). The computed skin-friction coefficients and the roughness functions show good agreement with experimental results. The double-averaging methodology is used to investigate mean velocity, Reynolds stresses, dispersive flux and mean momentum balance. The roll-up of the shear layer on the roughness crests is identified as a primary contributor to the wall-normal momentum transfer. The mean-square pressure fluctuations increase in the roughness layer and collapse onto their smooth-wall levels away from the wall. The dominant source terms in the pressure Poisson equation are examined. The rapid term shows that high pressure fluctuations observed in front of and above the roughness elements are mainly due to the attached shear layer formed upstream of the protrusions. The contribution of the slow term is smaller. The slow term is primarily increased in the troughs and in front of the roughness elements, corresponding to the occurrence of quasi-streamwise vortices and secondary vortical structures. The mean wall shear on the rough surface is highly correlated with the roughness height, and depends on the local roughness topography. The probability distribution function of wall shear stress fluctuations is consistent with higher velocities at roughness crests and reverse flow in the valley regions. Extreme events are more probable due to the roughness. Events with comparable magnitudes of the streamwise and spanwise wall shear stress occur more frequently, corresponding to a more isotropic vorticity field in the roughness layer.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/508C6FDFA9B22E9014B35A549889F327/S0022112020008745a.pdf/div-class-title-direct-numerical-simulation-of-turbulent-channel-flow-over-random-rough-surfaces-div.pdf

Direct numerical simulation of turbulent channel flow over random rough surfaces

This paper presents experimental measurements of the flow field in a Low-speed Turbine Cascade using a stereoscopic particle-image velocimetry (SPIV). During the measurements, a pair of frame-straddling-based CCD cameras were configured at different sides of the laser light sheet, and appropriate tracing particles (DEHS) were employed. The measurements were conducted at the incidence angle of 0 degree and exit Reynolds number of 1.7 × 105 with the tip clearance 1.18% of blade chord. The tip flow features, such as the evolution and breakdown of tip leakage vortex, the horseshoe vortex, turbulence characteristics of tip leakage flow, were studied for the flow field analysis. The results showed that the tip leakage flow/vortex mainly dominate flow fields in the tip region. The tip leakage vortex performs as a concentrated vortex before its breaking down and splitting into small vortices. The highest turbulence intensity mainly occurs in the tip region along with the trajectory of tip leakage vortex, and when the vortex breaks down, the turbulence intensity reduces rapidly. Additionally, the SPIV with this configuration also shows an advantage in investigating the flow structures and mechanism inside the turbine cascade.

Stereoscopic PIV measurements of the flow field in a turbine cascade

To explore the effects of airfoil-probe tubes and its installment position on the flow field of the compressor cascade, and find out the mechanism that how the airfoil-probes affect the aerodynamic characteristics of the compressor cascade, this paper performed both numerical and experimental works on the same compressor cascade. The experiment mainly focused on the cases of low Mach number (Ma = 0.1), and cases with different Mach numbers (0.1, 0.3, 0.7) and different incidence angles (-5, 0, 5) are investigated by the numerical method. The case without the airfoil-probe tube was referenced as the baseline, and other three cases with the airfoil-probe tubes installed in different chordwise positions (30%, 50%, 70% of the chord length) were studied. The diameter of the airfoil-probe tube is 3mm, which is configured as 300% amplification of some particular airfoil-probe according to the geometrical similarity principle. The results show that the airfoil-probe tubes have a negative influence on the flow capacity of the cascade at all investigation points. The separations and the large scale streamwise vortices that induced by the airfoil-probe tube on the pressure side cause most the losses at the high Mach number. The influence of the airfoil-probe tube on the flow field in the vicinity of the pressure side surface is local separation at the low Mach number. The airfoil-probe tubes also have a clearly effect on the leakage flow. It decreases the mass flow of the leakage flow and weakens the intensity of the leakage vortex, but enlarges the influence area. The total pressure loss of the case that the tube is installed at the half chordwise position is generally lower than other cases especially at the high Mach number, it can even decrease the losses compared with the basic case.

Effects of airfoil-probe tubes on the flow field of a compressor cascade

Direct numerical simulation (DNS) of flow in a turbulent channel with a random rough wall is performed at $Re_{\tau}=400$ and $600$. The roughness geometry corresponds to the experiments of Flack and Schultz (personal communication). The rough surface is used on the bottom wall of the channel. DNS of a smooth channel flow and a rod-roughened channel flow are also performed at $Re_{\tau}=400$ for validation and comparison. The skin friction coefficient of the random-rough channel shows good agreement with the experimental results of Flack and Schultz. Due to the roughness, the slip velocity on the rough wall increases while the mean velocity decreases in the log-law region. The streamwise and spanwise velocity fluctuations are enhanced near the rough wall. The pressure fluctuations show a significant increase in the roughness layer and exhibit a good collapse with the smooth wall in the outer layer. The streamwise mean momentum balance shows that pressure and viscous stress gradients are induced in the roughness layer, and the gradients are amplified at higher $Re_{\tau}$. The statistics of wall-shear stress fluctuations in the peak (above the mean height location) and valley (below the mean height location) regions are examined. The results indicate that reverse flow mainly occurs in the valley regions of the random roughness, and is enhanced at higher $Re_{\tau}$, but is not as strong as the recirculation within the cavities in the rod-roughened channel. The probability distribution function of wall-shear stress shows a better collapse after subtracting the mean and normalizing by the root-mean-squared value. The distribution tail is widened by the random roughness, implying that the probability of extreme events is increased. The probability of extreme events in the random-rough channel increases with increasing $Re_{\tau}$, in accordance with previous studies on smooth-wall flows.

Rong Ma

Papers

Dynamic pressure blade type probe

Direct numerical simulation of turbulent channel flow over random rough surfaces

Stereoscopic PIV measurements of the flow field in a turbine cascade

Effects of airfoil-probe tubes on the flow field of a compressor cascade

Direct numerical simulation of turbulent channel flow over random rough surfaces