Showing papers by "Sergio Pirozzoli published in 2021"

One-point statistics for turbulent pipe flow up to

Abstract: We study turbulent flows in a smooth straight pipe of circular cross-section up to friction Reynolds number using direct numerical simulation (DNS) of the Navier-Stokes equations. The DNS results highlight systematic deviations from Prandtl friction law, amounting to approximately, which would extrapolate to approximately at extreme Reynolds numbers. Data fitting of the DNS friction coefficient yields an estimated von Karman constant, which nicely fits the mean velocity profile, and which supports universality of canonical wall-bounded flows. The same constant also applies to the pipe centreline velocity, thus providing support for the claim that the asymptotic state of pipe flow at extreme Reynolds numbers should be plug flow. At the Reynolds numbers under scrutiny, no evidence for saturation of the logarithmic growth of the inner peak of the axial velocity variance is found. Although no outer peak of the velocity variance directly emerges in our DNS, we provide strong evidence that it should appear at, as a result of turbulence production exceeding dissipation over a large part of the outer wall layer, thus invalidating the classical equilibrium hypothesis.

Reynolds stress scaling in the near-wall region of wall-bounded flows

Abstract: A new scaling is derived that yields a Reynolds-number-independent profile for all components of the Reynolds stress in the near-wall region of wall-bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behaviour.

Towards the ultimate regime in Rayleigh–Darcy convection

Abstract: Numerical simulations are used to probe Rayleigh–Darcy convection in fluid-saturated porous media towards the ultimate regime. The present three-dimensional dataset, up to Rayleigh–Darcy number sequestration.

Reynolds-Averaged Numerical Simulations of Conical Shock-Wave/Boundary-Layer Interactions

Abstract: We carry out a parametric study of conical shock-wave/turbulent boundary-layer interactions by means of numerical simulation of the Reynolds-averaged Navier–Stokes (RANS) equations, with the eventu...

Crossflow effects on shock wave/turbulent boundary layer interactions

Abstract: The effects of crossflow on the interaction between an impinging shock wave and a high-speed turbulent boundary layer are investigated using direct numerical simulations of statistically two-dimensional, three-component flow. The leading-order effect of crossflow is increased size and strength of the separation bubble, with upstream and downstream displacement of the separation and reattachment points, respectively. This effect is traced to retarded growth of the shear layer surrounding the separation bubble, with associated reduction of the turbulent shear stress. Genuinely, three-dimensional effects are observed in the interaction and in the downstream recovery zone, with mean flow direction changing both in the longitudinal and wall-normal directions. Three-dimensional, non-equilibrium effects yield substantial misalignment between turbulent stresses and mean strain rate, thus providing a challenging benchmark for the development and validation of turbulence models for compressible flows.

Influence of corner angle in streamwise supersonic corner flow

Abstract: We use the Reynolds-averaged Navier–Stokes (RANS) equations with a full Reynolds stress model (RSM) to study the effect of the corner angle in supersonic corner flow. RANS data are compared to reference direct numerical simulation of fully developed a square duct flow, which support predictive capability of secondary flows from Stress-ω RSM. We then carry out a parametric study by changing the corner angle in the range θ = 45 °– 135 °, focusing on the effect on the mean streamwise and secondary flow. The maximum strength of the secondary flows of about 0.015 u ∞ occurs for θ = 90 °, which is similar to what is found in fully developed square ducts. Secondary eddies have approximately unit aspect ratio, and they maintain their shape for different corner angles by translating in the direction parallel to the closest wall. As a result, the position of the vortex center can be described by a simple geometrical transformation of the wall-parallel coordinate. We find that small corner angles are responsible for locally relaminarization flow at the corner, but otherwise the mean streamwise velocity profiles transformed according to van Driest following the canonical law-of-the-wall.

Reynolds number trends in turbulent pipe flow: a DNS perspective

Abstract: We study turbulent flows in a smooth straight pipe of circular cross--section up to $Re_{\tau} \approx 6000$ using direct--numerical-simulation (DNS) of the Navier--Stokes equations. The DNS results highlight systematic deviations from Prandtl friction law, amounting to about $2\%$, which would extrapolate to about $4\%$ at extreme Reynolds numbers. Data fitting of the DNS friction coefficient yields an estimated von Karman constant $k \approx 0.387$, which nicely fits the mean velocity profile, and which would restore universality of canonical wall-bounded flows. The same constant also applies to the pipe centerline velocity, thus providing support for the claim that the asymptotic state of pipe flow at extreme Reynolds numbers should be plug flow. At the Reynolds numbers under scrutiny, no evidence for saturation of the logarithmic growth of the inner peak of the axial velocity variance is found. Although no outer peak of the velocity variance directly emerges in our DNS, we provide strong evidence that it should appear at $Re_{\tau} \gtrsim 10^4$, as a result of turbulence production exceeding dissipation over a large part of the outer wall layer, thus invalidating the classical equilibrium hypothesis.

Reynolds stress scaling in the near-wall region of wall-bounded flows

Abstract: A new scaling is derived that yields a Reynolds number independent profile for all components of the Reynolds stress in the near-wall region of wall bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behavior.

WP-1 Reference Cases of Laminar and Turbulent Interactions

Abstract: In order to be able to judge the effectiveness of transition induction in WP-2, reference flow cases were planned in WP-1. There are two obvious reference cases—a fully laminar interaction and a fully turbulent interaction. Here it should be explained that the terms “laminar” and “turbulent” interaction refer to the boundary layer state at the beginning of interaction only. There are two basic configurations of shock wave boundary layer interaction and these are a part of the TFAST project. One is the normal shock wave, which typically appears at the transonic wing and on the turbine cascade. The characteristic incipient separation Mach number range is about M = 1.2 in the case of a laminar boundary layer and about M = 1.32 in the case of turbulent boundary layer. The second typical flow case is the oblique shock wave reflection. The most characteristic case in European research is connected to the 6th FP IP HISAC project concerning a supersonic business jet. The design speed of this airplane is M = 1.6. Therefore the TFAST consortium decided to use this Mach number as the basic case. Pressure disturbance at this Mach number is not very high and can be compared to the disturbance of the normal shock at the incipient separation Mach number mentioned earlier. As mentioned earlier, shock reflection at M = 1.6 may be related to incipient separation. Therefore two additional test cases were planned with different Mach numbers. ITAM conducted an M = 1.5 test case, and TUD an M = 1.7 test case. These partners have also previously made very specialized and successful contributions to the UFAST project.

WP-5 External Flows—Wing

Abstract: Study of transition location effect (from natural transition to fully turbulent) on separation size, shock structure and unsteadiness was the focus of this WP. Boundary layer tripping (by wire or roughness) and flow control devices (VG) were used for boundary layer transition induction. Although this type of flow field had been studied widely in the past, there remains considerable uncertainty on the effects of transition on transonic aerofoil performance. In particular it is not known how close to the shock location transition has to occur to avoid detrimental effects associated with laminar shock-induced separation. Furthermore, it was unclear how best to provoke transition on an airfoil featuring significant laminar flow and how close to the shock this needs to be performed. Finally, current CFD methods are particularly challenged by such transitional flows. In this work package some of the findings from the basic research performed in other WPs was applied. Specialized large-scale transonic wind tunnels running cost is very high therefore using such facilities is not appropriate for upstream research programs such as TFAST. Therefore we have used existing wind tunnels within our consortium. One of these is a transonic test section at UCAM where laminar and transitional profiles were studied previously at Reynolds numbers up to 2 million (based on chord length). This wind tunnel allowed basic investigations of the transition location effects on a shock induced separation and unsteadiness for a relatively large number of parameters. A larger wind tunnel at Institute of Aviation in Warsaw was used, which enabled the investigation of a much larger aspect ratio profile. In this facility it was possible to measure a whole force polar up to and including the buffet boundary. The research was carried out for the natural b/l transition location as well as different methods of tripping.

One-point statistics for turbulent pipe flow up to $Re_{\tau} \approx 6000$

Abstract: We study turbulent flows in a smooth straight pipe of circular cross--section up to $Re_{\tau} \approx 6000$ using direct--numerical-simulation (DNS) of the Navier--Stokes equations. The DNS results highlight systematic deviations from Prandtl friction law, amounting to about $2\%$, which would extrapolate to about $4\%$ at extreme Reynolds numbers. Data fitting of the DNS friction coefficient yields an estimated von Karman constant $k \approx 0.387$, which nicely fits the mean velocity profile, and which supports universality of canonical wall-bounded flows. The same constant also applies to the pipe centerline velocity, thus providing support for the claim that the asymptotic state of pipe flow at extreme Reynolds numbers should be plug flow. At the Reynolds numbers under scrutiny, no evidence for saturation of the logarithmic growth of the inner peak of the axial velocity variance is found. Although no outer peak of the velocity variance directly emerges in our DNS, we provide strong evidence that it should appear at $Re_{\tau} \gtrsim 10^4$, as a result of turbulence production exceeding dissipation over a large part of the outer wall layer, thus invalidating the classical equilibrium hypothesis.

WP-2 Basic Investigation of Transition Effect

Abstract: An important goal of the TFAST project was to study the effect of the location of transition in relation to the shock wave on the separation size, shock structure and unsteadiness of the interaction area. Boundary layer tripping (by wire or roughness) and flow control devices (Vortex Generators and cold plasma) were used for boundary layer transition induction. As flow control devices were used here in the laminar boundary layer for the first time, their effectiveness in transition induction was an important outcome. It was intended to determine in what way the application of these techniques induces transition. These methods should have a significantly different effect on boundary layer receptivity, i.e. the transition location. Apart from an improved understanding of operation control methods, the main objective was to localize the transition as far downstream as possible while ensuring a turbulent character of interaction. The final objective, involving all the partners, was to build a physical model of transition control devices. Establishing of such model would simplify the numerical approach to flow cases using such devices. This undertaking has strong support from the industry, which wants to include these control devices in the design process. Unfortunately only one method of streamwise vortices was developed and investigated in the presented study.