Comparison of Computational and Experimental Unsteady Cavitation on a Pitching Foil

1989 ◽  
Vol 111 (3) ◽  
pp. 290-299 ◽  
Author(s):  
F. Stern
Keyword(s):  
Experimental Data ◽  
Cavity Length ◽  
Major Axis ◽  
Computational Method ◽  
Cavity Surface ◽  
Surface Boundary ◽  
Nonlinear Method ◽  
Sheet Cavitation ◽  
Reduced Frequency ◽  
Surface Boundary Conditions

Comparisons are made between a nonlinear method for predicting unsteady sheet cavitation and available experimental data for a pitching foil for the purposes of verifying the calculations and to further analyze the flow. A dynamical approach is employed in which the form of the instantaneous cavity surface is modeled as a semiellipse. The cavity length (major axis), thickness (semiminor axis), and position are determined such that the nonlinear cavity-surface boundary conditions are satisfied approximately. The pressure on the instantaneous cavity surface is prescribed using an unsteady thick-foil potential-flow method based on Green’s second identity. The computational method yields best results in predicting the cavity dynamics, but underpredicts the cavity length. For fixed cavitation number, mean foil angle, and pitch amplitude, the cavity dynamics, such as maximum cavity size and cavity surface behavior, are shown to depend on the ratio of the cavity natural frequency for the foil fixed at the maximum pitch amplitude to the foil reduced frequency. For a certain value of this ratio, the cavitation response is shown to be a minimum. The experimental results confirm the computational trends up to the point that experimental data were obtained.

Numerical Analysis of Super-Cavitating Flow Around a Two-Dimensional Cavitator Geometry

2011 ◽  
Author(s):  
Sunho Park ◽  
Shin Hyung Rhee
Keyword(s):  
Experimental Data ◽  
High Speed ◽  
Cavity Length ◽  
Stokes Equations ◽  
Cavitating Flow ◽  
The Body ◽  
Computational Method ◽  
Computational Results ◽  
Two Dimensional ◽  
Cell Counts

Mostly for military purposes, which require high speed and low drag, super-cavitating flows around under-water bodies have been an interesting, yet difficult research subject for many years. In the present study, high speed super-cavitating flow around a two-dimensional symmetric wedge-shaped cavitator was studied using an unsteady Reynolds-averaged Navier-Stokes equations solver based on a cell-centered finite volume method. To verify the computational method, flow over a hemispherical head-form body was simulated and validated against existing experimental data. Through the verification tests, the appropriate selection of domain extents, cell counts, numerical schemes, turbulence models, and cavitation models was studied carefully. A cavitation model based on the two-phase mixture flow modeling was selected with the standard k-epsilon model for turbulence closure. The cavity length, surface pressure distribution, and the flow velocity at the interface were compared with experimental data and analytic solutions. Various computational conditions, such as different wedge angles and caviation numbers, were considered for super-cavitating flow around the wedge-shaped cavitator. Super-cavitation begins to form in the low pressure region and propagates downstream. The computed cavity length and drag on the body were compared with analytic solution and computational results using a potential flow solver. Fairly good agreement was observed in the three-way comparison. The computed velocity on the cavity interface was also predicted quite closely to that derived from the Bernoulli equation. Finally, comparison was made between the computational results and cavitation tunnel test data, along with suggestions for cavitator designs.

Analysis of Supersonic Turbulent Boundary Layers Over Rough Surfaces Using the K-Omega and Stress-Omega Models

2005 ◽  
Author(s):  
Guanghong Guo ◽  
M. A. R. Sharif
Keyword(s):  
Experimental Data ◽  
High Speed ◽  
Rough Surfaces ◽  
Computational Cost ◽  
Reynolds Stress Model ◽  
Surface Boundary ◽  
Mean Flow ◽  
Surface Boundary Conditions ◽  
The Mean ◽  
Layer Flow

In this study, the influence of surface roughness in the prediction of the mean flow and turbulent properties of a high-speed supersonic (M = 2.9, Re/m = 2.0e7) turbulent boundary layer flow over a flat plate is performed using the k-ω and the stress-ω models. Six wall topologies, including a smooth and five rough surfaces consisting of three random sand-grain plates and two uniformly machined plates were tested. Experimental data are available for these configurations. It is observed that, for smooth surface, both k-ω and stress-ω models perform remarkably well in predicting the mean flow and turbulent quantities in supersonic flow. For rough surfaces, both models matched the experimental data profiles fairly well for lower values of the roughness height. Overall, the k-ω model performed better than the stress-ω model. The stress-ω model did not show any strong advantages to make up for the extra computational cost associated with a Reynolds stress model. The simulation results indicated that the prescription for the surface boundary conditions for ω in both models, especially for the stress-ω model, need to be refined encountering high roughness numbers and reconsidered to include the geometric factor.

Modeling Active Layer Depth of Permafrost Changing Surface Boundary Conditions

2020 ◽  
Author(s):  
MODI ZHU ◽  
Jingfeng Wang ◽  
Husayn Sharif ◽  
Valeriy Ivanov ◽  
Aleksey Sheshukov
Keyword(s):  
Boundary Conditions ◽  
Active Layer ◽  
Surface Boundary ◽  
Layer Depth ◽  
Surface Boundary Conditions

Explanation of the Influence of Inflow Air Content on the Lift of Cavitating Hydrofoils

2018 ◽  
Vol 140 (8) ◽  
Author(s):  
Eduard Amromin
Keyword(s):  
Experimental Data ◽  
Theoretical Study ◽  
Lift Coefficient ◽  
Cavity Surface ◽  
Air Bubbles ◽  
Incoming Flow ◽  
Fluid Density ◽  
Air Content ◽  
Theoretical Results ◽  
Good Agreement

According to several known experiments, an increase of the incoming flow air content can increase the hydrofoil lift coefficient. The presented theoretical study shows that such increase is associated with the decrease of the fluid density at the cavity surface. This decrease is caused by entrainment of air bubbles to the cavity from the surrounding flow. The theoretical results based on such explanation are in a good agreement with the earlier published experimental data for NACA0015.

Loss Generation in Transonic Turbine Blading

2017 ◽  
Author(s):  
Penghao Duan ◽  
Choon S. Tan ◽  
Andrew Scribner ◽  
Anthony Malandra
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Mach Number ◽  
High Speed ◽  
Turbine Blades ◽  
Surface Boundary ◽  
Loss Model ◽  
Exit Mach Number ◽  
Loss Characteristic ◽  
Shock Loss

The measured loss characteristic in a high-speed cascade tunnel of two turbine blades of different designs showed distinctly different trend with exit Mach number ranging from 0.8 to 1.4. Assessments using steady RANS computation of the flow in the two turbine blades, complemented with control volume analyses and loss modelling, elucidate why the measured loss characteristic looks the way it is. The loss model categorizes the total loss in terms of boundary layer loss, trailing edge loss and shock loss; it yields results in good agreement with the experimental data as well as steady RANS computed results. Thus RANS is an adequate tool for determining the loss variations with exit isentropic Mach number and the loss model serves as an effective tool to interpret both the computational and experimental data. The measured loss plateau in Blade 1 for exit Mach number of 1 to 1.4 is due to a balance between a decrease of blade surface boundary layer loss and an increase in the attendant shock loss with Mach number; this plateau is absent in Blade 2 due to a greater rate in shock loss increase than the corresponding decrease in boundary layer loss. For exit Mach number from 0.85 to 1, the higher loss associated with shock system in Blade 1 is due to the larger divergent angle downstream of the throat than that in Blade 2. However when exit Mach number is between 1.00 and 1.30, Blade 2 has higher shock loss. For exit Mach number above around 1.4, the shock loss for the two blades is similar as the flow downstream of the throat is completely supersonic. In the transonic to supersonic flow regime, the turbine design can be tailored to yield a shock pattern the loss of which can be mitigated in near equal amount of that from the boundary layer with increasing exit Mach number, hence yielding a loss plateau in transonic-supersonic regime.

Prediction of Cavitation Inception Within Regions of Flow Separation

2017 ◽  
Vol 140 (1) ◽  
Author(s):  
Eduard Amromin
Keyword(s):  
Experimental Data ◽  
Turbulent Flow ◽  
Flow Separation ◽  
Computational Method ◽  
Average Flow ◽  
Cavitation Inception ◽  
The Core ◽  
Good Agreement

Cavitation within regions of flow separation appears in drifting vortices. A two-part computational method is employed for prediction of cavitation inception number there. The first part is an analysis of the average flow in separation regions without consideration of an impact of vortices. The second part is an analysis of equilibrium of the bubble within the core of a vortex located in the turbulent flow of known average characteristics. Computed cavitation inception numbers for axisymmetric flows are in the good agreement with the known experimental data.

The Impact of Inlet Distortion and Reduced Frequency on the Performance of Centrifugal Compressors

2018 ◽  
Vol 141 (2) ◽  
Author(s):  
A. Grimaldi ◽  
V. Michelassi
Keyword(s):  
Experimental Data ◽  
Liquefied Natural Gas ◽  
Design Phase ◽  
Centrifugal Compressors ◽  
Axial Compressors ◽  
Inlet Distortion ◽  
Side Stream ◽  
Reduced Frequency ◽  
The Impact

This paper discusses the impact of inlet flow distortions on centrifugal compressors based upon a large experimental data base in which the performance of several impellers in a range of corrected flows and corrected speeds have been measured after been coupled with different inlet plenums technologies. The analysis extends to centrifugal compressor inlets including a side stream, typical of liquefied natural gas applications. The detailed measurements allow a thorough characterization of the flow field and associated performance. The results suggest that distortions can alter the head by as much as 3% and efficiency of around 1%. A theoretical analysis allowed to identify the design features that are responsible for this deviation. In particular, an extension of the so-called “reduced-frequency,” a coefficient routinely used in axial compressors and turbine aerodynamics to weigh the unsteadiness generated by upstream to downstream blade rows, allowed to determine a plenum-to-impeller reduced frequency that correlates very well with the measured performance. The theory behind the new coefficient is discussed together with the measurement details and validates the correlation that can be used in the design phase to determine the best compromise between the inlet plenum complexity and impact on the first stage.

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