Two-dimensional Diffuser Research Articles

Performance Analysis of Two-Dimensional Plane Diffusers at a Moderate Reynolds Number by Means of Unsteady RANS

Performance Analysis of Two-Dimensional Plane Diffusers at a Moderate Reynolds Number by Means of Unsteady RANS

The design of two-dimensional elastic diffusers

The ability to design surfaces with specific scattering properties has widespread applicability. In this article, we leverage a stochastic method for designing randomly rough, two-dimensional surfaces with a prescribed mean angular intensity distribution. Analytical and numerical techniques are implemented for a linearly elastic medium-vacuum interface, to generate band-limited diffusers with square and elliptical domains of scattering. Longitudinal incidence is assumed, and the theory allows for bulk wave mode conversions. Multivalued, discontinuous surfaces are formulated, and it is shown that the theoretical model may be satisfied independently of the facet shape utilised, with square and hexagonal discretisations given as examples. In the context of elastic wave scattering, the surfaces presented in this article may have applications in the calibration of scatterometers for wave manipulation as well as for crack detection using ultrasound.

Design and performance of an ultra-compact, low-speed, low turbulence level, wind tunnel for aerodynamic and animal flight experiments

We present a novel closed-circuit ultra-compact wind tunnel with an 8:1 contraction ratio and high flow quality. Its overall footprint area is less than half that of a conventional tunnel with the same test section size and same contraction ratio, enabling significantly smaller material and construction costs. The tunnel’s key features which enable the small footprint include a two-dimensional main diffuser, a minimum-length contraction, and expanding turning vanes with a 1.167:1 ratio in corner two and an aggressive 1.875:1 ratio in corner four. Separation in the latter is prevented using a screen and honeycomb integrated into each vane passage—the first time this has been used in a wind tunnel. The tunnel exhibits excellent flow quality with less than pm 1% mean flow variation in the test section core and a freestream turbulence level of 0.03% at 12 m/s over a 4 Hz–20kHz bandwidth.Graphical abstract

Design of Curved Annular Diffusers II: Using Splitter Blades

Abstract The use of splitter blade as a passive flow control mechanism in the design of separation free aggressive annular diffuser is explored through computational fluid dynamics simulations. The fundamental working principle of a splitter blade in case of a two-dimensional rectangular diffuser and an annular diffuser is discussed. The effects of splitter blade configuration on the end-wall adverse pressure gradients are discussed. An aggressive diffuser design space is identified with ducts of maximum slope of 50 deg and maximum divergence angle between the outer and inner walls of 10 deg for axial-length to inlet height ratio ranging from 1.25 to 2.5. One or more splitter blades are employed in the duct to eliminate flow separation for all the ducts in the aggressive design space considered. Requirement of number of splitter blades in the aggressive design space is demarcated. Performance charts for the ducts in this aggressive design space are also established.

An experimental study on acoustical performance of cross rib diffuser

The preset paper proposes a novel acoustic diffuser which we call cross rib diffuser (CRD) and investigates its acoustical performance in rooms experimentally. CRD consists of overlapping two one-dimensional periodic rib diffuser (OPRD) with different structural configurations. CRD can achieve high scattering coefficient with wider frequency band than OPRD. Moreover, unlike other diffusers with high scattering property such as metadiffuser and two-dimensional quadratic diffuser, CRD keeps simple and familiar design of OPRD suitable for use in various architectural spaces. In the paper, we firstly evaluated random-incidence scattering coefficient of CRD using 1/5 scaled reverberation room. Then, random-incidence absorption coefficient was measured in 1/1 reverberation room. Finally, an implementation experiment was conducted to examine applicability of CRD in improving acoustics in small meeting room with small absorption treatments. The results indicated that CRD reduced EDT and reverberation time, and increased D50 more than JND values. Additionally, CRD improved reverberation and speech intelligibility more significantly than OPRD with same installation area.

Diffuser Optimization Using Computational Fluid Dynamics and Micro-Genetic Algorithms.(Dept.M)

An approach for the optimization of turbulent flow in diffusers is presented. A methodology is developed to integrate a finite volume-based computational fluid dynamics (CFD) model and an optimization tool uses micro-genetic algorithms (u GA). The CFD model is based on the Reynolds-averaged Navier-Stokes equations, with the standard k-e closure turbulence model. This methodology is tested on two cases. The first is the estimation of the conical diffuser length, which gives the maximum pressure recovery coefficient, for a given diffuser area ratio. Good agreement between the computational and experimental results is obtained. The second case is the optimization through wall contouring of a given two-dimensional diffuser area ratio and length ratio. The results indicate that the diffuser performance can be improved by this method.

Separation Control and Heat Transfer over a Fence inside an Asymmetric Diffuser (Dept.M)

The present work is concerned with an experimental investigation of flow and Heat transfer through a fence located in an asymmetric plane two-dimensional diffuser. The average velocity and fluctuation fields have been measured with a constant temperature hot-wire anemometer (CTA). However, pressures in reversed flow regions have been measured using 5-hole-probe. Of special interest is the influence of the fence on measuring the velocity and the pressure of the flow in relation with heal faster parameters at five well-chosen stations of the diffuser. The obtained experimental data are used to provide a comparison between the cases of using and not using the fence for the sake of separation control and Consequently the determination of the variation of Stanton and Nusselt numbers, particularly in the separated regions. Also, the investigation is reported for some attributes about the sensitivity of the separated and reversed flow regions, that represent a range of flows in which the controlling mechanisms are largely unknown

Acoustic metasurfaces as sound diffusers and diffsorbers

Acoustic metasurfaces (AMS) may offer additional design space for the creation of sound diffusers and diffsorbers. We present the design and experimental characterization of an AMS created to diffuse sound fields using a treatment of uniform thickness. The AMS unit cell designs were chosen to minimize viscous losses while enabling control of reflected phase at each point of the AMS. The design geometry was then optimized with a parametric sweep of unit cell dimensions using finite element (FE) models where the target phase shifts are associated with a one- and two-dimensional quadratic residue diffuser (QRD). Samples were fabricated using additive manufacturing and impedance tube measurements were conducted to validate FE predictions. Modeled and measured reflection coefficients were shown to be in good agreement. The scattered field was then measured in an anechoic chamber using the logarithmic frequency modulated chirp excitation and the deconvolution methods to provide narrowband and broadband estimates of sound diffusion performance. Overall performance is presented for narrow-band and broadband considerations. The AMS response is compared and contrasted with traditional QRD designs, highlighting differences in frequency- and angle-dependence as well as temporal diffusion and absorption. [S.N.L. is managed and operated by NTESS under DOE NNSA Contract No. DE-NA0003525.]

A Numerical Study on the Flow Mechanism of Performance Improvement of a Wide-Angle Diffuser by Inserting a Short Splitter Vane

Usage of a wide-angle diffuser may result in unfavorable separated flow and a significant diffuser loss. To improve the performance of the diffusers, inserting short splitter vanes is known as a useful method that has been demonstrated experimentally. Regarding the role of the vane in the diffuser flow, Senoo & Nishi (1977) qualitatively explained that the lift force acting on the vane should be a key factor. However, its quantitative verification remains since then. To challenge this issue, numerical simulations of incompressible flow in a wide angle of 28° two-dimensional diffuser with and without a short splitter vane were conducted in the present study. An improvement of pressure-recovery by the vane and oscillatory flows in the diffuser are reasonably reproduced from comparison with the experimental results made by Cochran & Kline (1958). It is also found that the lift force acting on the vane varies periodically in an opposite phase with the detachment point moved back and forth on a diverging wall, since one vane is not sufficient to fully suppress the flow separation that occurred on the wall and the incoming main-flow shifts toward the other diverging wall in the diffuser. Thus, as a role of splitter vane in the diffuser, “the lift force of the vane is a key factor” may be quantitatively verified from the present numerical simulation. Further, it is confirmed by the local loss analysis that the turbulent kinetic energy production observed in mixing layers contributes most of the loss in the diffuser. Consequently, the present numerical technique may be usable to investigate the flow character in a diffuser with splitter vanes at a design stage.

Development of k-R turbulence model for wall-bounded flows

A new low-Reynolds-number turbulence model is formulated based on the turbulent kinetic energy k and eddy-viscosity parameter R=k2/ϵ. In the derivation, most of the diffusion terms emerging from the transformation have been preserved in the proposed model, maintaining the closest relationship with its parent k-ϵ model; the near-wall viscous balance is guaranteed by including a far-wall damping function. The turbulent structure parameter (i.e., Bradshaw-relation) that may enhance the predicted accuracy for non-equilibrium flows, is incorporated into the current model. The coefficients and functions are constructed such as to preserve the anisotropic characteristics of turbulence, encountered in rotational and irrotational flows. Fully-developed turbulent channel and flat-plate flows are computed to validate the model ability in replicating the near-wall turbulence. Two-dimensional asymmetric plane diffuser and airfoil flows are simulated to verify the model accuracy in capturing non-equilibrium flows with separation and reattachment. Computation of a three-dimensional wing with shock-wave is presented, rectifying the model competency in predicting large adverse-pressure-gradient flows. Furthermore, model predictions are compared with other two frequently-used turbulence models; a good correlation is obtained between the current model and experimental data.

Data-driven uncertainty quantification for Formula 1: Diffuser, wing tip and front wing variations

This work introduces a new uncertainty quantification method to better deal with scarce data and long simulation run times in Formula 1 design. Race cars are produced in low quantities and for maximum performance. Thus, their designing process is characterised by manufacturing data shortage and complex Computational Fluid Dynamics simulations with long run times. Their car aerodynamics is subject to many random variables that introduce uncertainty into the down-force and drag performance, such as variations in ride height, front wing direction and pitch angle. To accurately predict the car performance during a race, it is important to study the effect of these random variables. This assessment cannot be performed with the standard deterministic Computational Fluid Dynamics approaches used in Formula 1. Even with regard to stochastic approaches, no efficient method has so far been suggested that addresses the problem of data scarcity. The reason for this is that most efficient uncertainty quantification methods fit probability distributions to the scarce data. It is shown in this work that probability distribution fitting can create a significant error using a simple two-dimensional diffuser example. Subsequently, the use of a new data-driven Polynomial Chaos method and its sparse multi-dimensional extension is suggested and demonstrated for Formula 1 to reduce such errors. This method allows to avoid distribution fitting because it is based on pure data. SAMBA’s general formulation also makes it easier to combine any possible inputs within a sparse description for problems with many variables. SAMBA is applied to two realistic car three-dimensional Computational Fluid Dynamics simulations: a NACA 0012 tip wing and the front part of a Formula 1 car. The probabilistic variations of the lift and drag of these two configurations are calculated using SAMBA and shown to be significant.

Favre-Averaged Nonlinear Harmonic Method for Compressible Periodic Flows

The treatment of compressible flows subject to periodic disturbances of finite amplitude with nonlinear harmonic (NLH) methods requires a careful choice of averaging procedures and variable sets to average. A judicious choice of averaging procedure for the unsteady Navier–Stokes equations and its associated variable set is one that preserves certain properties of the flow of particular interest and also leads to a simple formulation of the equations governing the time-mean flow. This paper studies the time-averaged Navier–Stokes equations with different averaging procedure and variable set, and presents a new formulation of the NLH method based on Favre averaging of the primitive variable set. The resulting formulation of the time-averaged Navier–Stokes equations has exactly the same form as the steady base flow plus contributions from deterministic fluxes and stresses. Favre averages of the primitive variables preserve the time average of the conserved flow variables. From a practical point of view, this can minimize the modifications to the baseline solver to implement the NLH method. The details of the method are presented, and the method is validated from a two-dimensional transonic diffuser to compressors in subsonic and transonic flow regimes. A good agreement with the unsteady simulations using time marching is observed for all cases.

Experimental investigation of coherent structures of a three-dimensional separated turbulent boundary layer

Coherent structures of a three-dimensional (3D) separation due to an adverse pressure gradient are investigated experimentally. The flow set-up consists of a flat plate to develop a turbulent boundary layer upstream of an asymmetric two-dimensional diffuser with one diverging surface. The diffuser surface has an initial mild curvature followed by a flat section where flow separation occurs. The top and the two sidewalls of the diffuser are not equipped with any flow control mechanism to form a 3D separation. Planar particle image velocimetry (PIV) using four side-by-side cameras is applied to characterize the flow with high spatial resolution over a large streamwise-wall-normal field of view (FOV). Tomographic PIV (tomo-PIV) is also applied for volumetric measurement in a domain flush with the flat surface of the diffuser. The mean flow obtained from averaging instantaneous velocity fields of this intermittent unsteady flow appears as a vortex with an elliptical cross-section. The major axis of the ellipse is tilted with respect to the streamwise direction. As a result, the average velocity in the mid-span of the diffuser has an upstream forward flow and a downstream backward flow, separated by a point of zero wall shear stress. Sweep motions mainly carry out transport of turbulent kinetic energy upstream of this point, while ejections dominate at the downstream region. In the instantaneous flow fields, forward and backward flows have equivalent strength, and the separation front is extended in the spanwise direction. The conditional average of the separation instants forms a saddle-point structure with streamlines converging in the spanwise direction. Proper orthogonal decomposition (POD) of the tomo-PIV data demonstrates that about 42 % of the turbulent kinetic energy is present in the first pair of modes, with a strong spanwise component. The spatial modes of POD also show focus, node and saddle-point structures. The average of the coefficients of the dominant POD modes during the separation events is used to develop a reduced-order model (ROM). Based on the ROM, the instantaneous 3D separation over the diffuser is a saddle-point structure interacting with focus-type structures.

Numerical simulation of a viscoelastic RANS turbulence model in a diffuser

One of the newest viscoelastic RANS turbulence models for drag reducing flows with polymer additives is studied considering different rheological properties. A finitely extensible nonlinear elastic-Peterlin (FENE-P) constitutive model is used to describe the viscoelastic effect of the polymer solutions and the $$k - \varepsilon - \overline {{v^2}} - f$$ turbulence framework is applied for turbulence modelling. The geometry in this study is a twodimensional diffuser. The finite volume method (FVM) with a non-uniform collocated mesh is used to solve the momentum and constitutive equations. In order to evaluate the turbulence model, the flow is simulated with different parameters such as the Weissenberg number and the maximum polymer extensibility and compared with the experimental results qualitatively. The velocity profiles, pressure distribution, reattachment length, and the amount of the drag reduction predicted by the turbulence model are in line with the experimental results.

2D brine sewage after impinging on a shallow sea free surface

We consider the problem of the vertically upwards disposal of heavy brine sewage from a two-dimensional diffuser in a lighter, homogeneous, motionless and shallow ambient sea. The rejected high salinity water of seawater desalination plants for urban and agricultural uses is such a case of a two dimensional fountain. The disposal of brine sewage produces a negative buoyant jet due to its initial momentum, which impinges on the free surface, spreads laterally on it and then sinks downwards, because of the negative buoyancy. Laboratory experiments and dimensional considerations are used in this paper in order to investigate the spreading behavior (width) of the vertical fountain which impinges on the free surface of the shallow ambient fluid. The experimental results have been used to derive an equation relating the width at the free surface with the initial parameters of the flow. In addition, the experimentally measured dilution of the heavier brine sewage on the recipient’s surface is compared with the dilution which was calculated by a numerical simulation of a well-known commercial software package, CORJET (a CORMIX sub model).

A near-wall domain decomposition approach in application to turbulent flow in a diffuser

Turbulent flow in an asymmetric, two-dimensional diffuser is studied using a near-wall domain decomposition method and a k–ε turbulence model. A one-dimensional boundary layer equation is used to transfer the boundary conditions from the wall to an interface within the flow. The boundary conditions applied to the fluid velocity and turbulent kinetic energy are of Robin type. They are mesh independent and can account for arbitrary source terms. This approach avoids the computational expense of fully simulating the turbulent boundary layers. For the first time, the technique has been applied to modelling a separated flow with an unstructured code. It is shown how the interface boundary condition on the turbulent kinetic energy allows the recirculation region in the diffuser to be captured. In contrast, the standard wall function approach, based on the log law, fails to predict any recirculation region. The only parameter required to apply the domain decomposition method is a turbulent viscosity profile across the boundary layer. Three different profiles are used in this work. It is shown how making the turbulent viscosity a function of the pressure gradient improves flow predictions for the diffuser. The results demonstrate that the method is an efficient way to simulate the boundary layers in engineering problems that include complex geometries or separating flows.

Geometry optimization of the diffuser for the supersonic wind tunnel using genetic algorithm and adaptive mesh refinement technique

Design of two-dimensional supersonic diffusers as a part of the wind tunnel is investigated in this paper. A methodology based on the mixture of try-and-error method and optimization algorithm is developed to handle the design problem. In the first design step, using try-and-error approach, the main parameters related to the geometry of diffuser such as length, angle and area ratio between the throat and the outlet are determined assuming a diffuser with linear walls. The design criterion in this step is the fact that the shock wave should be created near the diffuser throat in order to benefit from the maximum efficiency of diffuser. In the second design step, considering the optimization methodology, it is tried to improve the optimum design obtained in the first step by modifying the wall point locations and keeping the rest of the geometry fixed. Hence, an optimization problem is defined to find the best curve for the diffuser wall instead of the a linear one used the in the first step. The objective function of this problem is to minimize the output Mach number using Genetic Algorithm (GA). The fluid flow is evaluated using the Euler equations in the conservative form where the Streamline-Upwind/Petrov–Galerkin (SUPG) finite element scheme is used to discretize the flow equations. In order to capture the flow solution around the shock waves accurately, the adaptive mesh refinement technique is coupled to the flow solution. The demonstrated results show the efficiency of the proposed method for designing supersonic diffusers.

A new algorithm for the implementation of wall-functions in high Reynolds number simulations

A new optimization algorithm based on a minimum residual technique is introduced to reduce the instability of numerical methods due to using wall functions to specify the boundary conditions for high Reynolds number flows. In the present article, four different models of the wall-function approximation are investigated for separated flow inside a two-dimensional asymmetric straight-walled diffuser. The equations of motion are closed with the κ– $$\varepsilon$$ turbulent model. The spatial discretization of the computational domain is performed using a finite element method, whereas the temporal discretization is based on a semi-implicit sequential scheme of finite differences. The pressure–velocity coupling is solved through a variation of the algorithm of Uzawa. Numerical noise resulting from the symmetric treatment of the convective fluxes is treated via a balance dissipation method. The non-linearities resulting from the wall-function explicit calculation are dealt with by a minimal residual method.

Stability analysis and control of the flow in a symmetric channel with a sudden expansion

The laminar flow in two-dimensional diffusers may produce either symmetric or nonsymmetric steady solutions, depending on the value of the Reynolds number as compared with some critical value. The stability properties of the flow are studied in the context of linear theory. In this context, a sensitivity analysis of the flow instability is carried out with respect to perturbations that may be produced by a realistic passive control, thus providing qualitative hints and quantitative information for the control design. Following the so-obtained information, a passive control is built by introducing a small cylinder in the flow with the aim of stabilizing the unstable symmetric flow configuration in the diffuser. The effectiveness of this control is finally assessed by direct numerical simulation. It is shown that the introduction of the cylinder, placed following the indications of the linear sensitivity analysis in the stable asymmetric flow configuration, allows a steady completely symmetric or less asymmetric flow to be recovered. The flow transient between the uncontrolled asymmetric solution and the symmetric controlled one is analyzed in terms of streamlines and vorticity evolution; the effects of the cylinder introduction on flow dissipation are also assessed.

Speckle reduction using orthogonal arrays in laser projectors

We propose using a two-level (-1 and +1 as variables) orthogonal array (OA) to generate a binary phase diffuser for speckle reduction in laser projection displays. Compared with the Hadamard matrix, the diffuser generated from OA is more flexible. The speckle contrast ratio (CR) when introducing the binary phase diffuser at an intermediate image plane within the projector is calculated, and the minimum speckle CR can be achieved by finite step change of the diffuser patterns. With Kronecker algebra, the two-dimensional diffuser can also be replaced by two one-dimensional diffusers with the same function, and it can be implemented into the laser projector electronically and easily.