Laminar separation point of flow on surface of symmetrical airfoil

2016 ◽  
Author(s):  
Vladimir A. Frolov
Keyword(s):  
Separation Point ◽  
Laminar Separation ◽  
Flow On Surface

A Prediction Model for Separated-Flow Transition

1998 ◽  
Author(s):  
Anca Hatman ◽  
Ting Wang
Keyword(s):  
Reynolds Number ◽  
Prediction Model ◽  
Flow Behavior ◽  
Separated Flow ◽  
Separation Point ◽  
Flow Transition ◽  
Laminar Separation ◽  
Free Stream Turbulence ◽  
Displacement Thickness ◽  
Transition Modes

The present study formulates an improved approach for analyzing separated-flow transition that differentiates between the transition process in boundary layers which are laminar at separation and those which are already transitional at separation. The paper introduces new parameters that are necessary in classifying separated-flow transition modes and in accounting for the concomitant evolution of transition in separated shear layer and the average effect of periodic separation bubble build-up and vortex shedding. At least three separated-flow transition modes are positively distinguished: (a) transitional separation, with the transition starting upstream of the separation point and developing mostly as natural transition, (b) laminar separation - short bubble mode, with the onset of transition induced downstream of the separation point by inflexional instability and with a quick transition completion, and (c) laminar separation - long bubble mode, with the onset of transition also induced downstream of the separation point by inflexional instability, and with the transition completion delayed. Passing from one mode to another takes place continuously through a succession of intermediate stages. The location of maximum bubble elevation has been proved to be the controlling parameter for the separated flow behavior. It was found that, downstream of the separation point, the experimental data expressed in terms of distance Reynolds number Rex can be correlated better than momentum or displacement thickness Reynolds number data. For each mode of separated-flow transition, the onset of transition, the transition length, and separated flow general characteristic are determined. This prediction model is developed mainly on low free-stream turbulence flat plate data and limited airfoil data. Extension to airfoils and high turbulence environment requires additional study.

Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble

2016 ◽  
Vol 798 ◽  
pp. 5-26 ◽  
Author(s):  
Andrea Sansica ◽  
Neil D. Sandham ◽  
Zhiwei Hu
Keyword(s):  
Boundary Layer ◽  
Linear Stability ◽  
Flow Instability ◽  
Separation Bubble ◽  
Low Frequency ◽  
Boundary Layer Transition ◽  
Separation Point ◽  
Oblique Shock Wave ◽  
Laminar Separation Bubble ◽  
Laminar Separation

Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low-frequency unsteadiness. A laminar boundary layer interacting with an oblique shock wave at $M=1.5$ is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency unsteadiness is found near the separation point with a peak at a Strouhal number of $0.04$, based on the separation bubble length. The appearance of the low-frequency unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary layer, with a strong response near the separation point. The existence of the unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.

The laminar separation of an incompressible fluid streaming past a smooth surface

1977 ◽  
Vol 356 (1687) ◽  
pp. 443-463 ◽  
Keyword(s):  
Pressure Gradient ◽  
Incompressible Fluid ◽  
Smooth Surface ◽  
Bluff Body ◽  
Stokes Equations ◽  
Adverse Pressure Gradient ◽  
Separation Point ◽  
Large Reynolds Number ◽  
Laminar Separation ◽  
Boundary Layer Problem

Sychev’s (1972) proposal, that in general the laminar separation and breakaway of an incompressible fluid streaming past a smooth surface (e. g. on a bluff body) takes place through a triple-deck structure around the separation point, is examined numerically in this paper. The proposed pattern for large Reynolds number ( Re ) flows is based on a modification of the classical Kirchhoff (1869) free streamline theory, in which a slight adverse pressure gradient is provoked in the inviscid motion immediately ahead of the breakaway. This pressure gradient is just enough to generate a triple-deck development closer to the separation point. The major task then is to decide whether or not a solution of the basic triple-deck problem exists, and is regular at separation, and if it is unique. The numerical in-vestigation, an iterative calculation of the relevant boundary layer problem, together with the potential flow relation between the unknown pressure and displacement, points fairly firmly to both the existence and uniqueness of a solution. Thus, for the bluff body problem when Re ≫1, the triple-deck determines exactly how far the separation point lies from the position implied by inviscid (Kirchhoff) theory. Comparisons with separating incompressible fluid motions determined numerically from the Navier-Stokes equations and measured experimentally give some support overall to the triple-deck description. For the flow past a circular cylinder the agreement in the variation of pressure and skin friction near separation is in general very encouraging, for Reynolds numbers as low as 30.

A Prediction Model for Separated-Flow Transition

1999 ◽  
Vol 121 (3) ◽  
pp. 594-602 ◽  
Author(s):  
A. Hatman ◽  
T. Wang
Keyword(s):  
Reynolds Number ◽  
Prediction Model ◽  
Flow Behavior ◽  
Separated Flow ◽  
Separation Point ◽  
Flow Transition ◽  
Laminar Separation ◽  
Free Stream Turbulence ◽  
Displacement Thickness ◽  
Transition Modes

The present study formulates an improved approach for analyzing separated-flow transition that differentiates between the transition process in boundary layers that are laminar at separation and those that are already transitional at separation. The paper introduces new parameters that are necessary in classifying separated-flow transition modes and in accounting for the concomitant evolution of transition in separated shear layer and the average effect of periodic separation bubble build-up and vortex shedding. At least three separated-flow transition modes are positively distinguished: (a) transitional separation, with the transition starting upstream of the separation point and developing mostly as natural transition, (b) laminar separation/short bubble mode, with the onset of transition induced downstream of the separation point by inflexional instability and with a quick transition completion, and (c) laminar separation/long bubble mode, with the onset of transition also induced downstream of the separation point by inflexional instability, and with the transition completion delayed. Passing from one mode to another takes place continuously through a succession of intermediate stages. The location of maximum bubble elevation has been proved to be the controlling parameter for the separated flow behavior. It was found that, downstream of the separation point, the experimental data expressed in terms of distance Reynolds number Rex can be correlated better than momentum or displacement thickness Reynolds number. For each mode of separated-flow transition, the onset of transition, the transition length, and separated flow general characteristic are determined. This prediction model is developed mainly on low free-stream turbulence flat plate data and limited airfoil data. Extension to airfoils and high turbulence environment requires additional study.

scholarly journals Separated-Flow Transition: Part 3 — Primary Modes and Vortex Dynamics

1998 ◽  
Author(s):  
Anca Hatman ◽  
Ting Wang
Keyword(s):  
Shear Layer ◽  
Boundary Layers ◽  
Separated Flow ◽  
Vortex Sheet ◽  
Separation Point ◽  
Transition To Turbulence ◽  
Flow Transition ◽  
Maximum Displacement ◽  
Laminar Separation ◽  
Separated Shear Layer

This paper clearly identifies the possible modes of transition in the separated boundary layers and their specific characteristics. This study distinguishes between the short and long bubbles primarily based on the separated flow structure. A hypothetical description of the vortex structure and evolution for each separated-flow transition mode is provided. The present approach in analyzing separated-flow transition is based on the assumption that the transition to turbulence in separated boundary layers is a result of the superposition of the effects of two different types of instability. The first type of instability is the Kelvin-Helmholtz (KH) instability. It occurs and develops in the shear layer at a specific location downstream of the separation point. The concentration of spanwise vorticity grows in time and remains in place through the vortex sheet roll-up mechanism. The roll-up vortex interacts with the wall and induces periodic ejection of near-wall fluid into the separated shear layer. The ejection process takes place at a location identifiable by the maximum displacement of shear layer, xMD. The second type of instability is the (convective) Tollmien-Schlichting (TS) instability. It originates in the boundary layer prior to the separation point and continues to evolve in the separated shear layer. The mechanism for the TS instability also leads to roll-ups, but it involves viscous tuning of the instability waves. Thus, the separated-flow transition is the result of spatially developing, often competing instabilities. The ejection induces the onset of transition for laminar short and long bubble modes of transition and controls the mid-transition point of transitional separation mode. The ejection may be accompanied by vortex shedding. Shedding occurs in the laminar separation - short bubble mode and occasionally in the transitional separation mode; however, it is not present in the laminar separation - long bubble mode of transition.

Measurements near a laminar separation point

1984 ◽  
Vol 138 ◽  
pp. 1-19 ◽  
Author(s):  
R. L. Varty ◽  
I. G. Currie
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Stokes Equations ◽  
Separation Point ◽  
Navier Stokes ◽  
Film Sensor ◽  
Laminar Separation ◽  
Boundary Layer Equations ◽  
Beam Laser ◽  
Dual Beam

Measurements in the neighbourhood of a laminar separation point at a high subcritical Reynolds number are reported. These results are used to test the validity of various theories relating to laminar separation. It is concluded that the boundary-layer equations are valid in the neighbourhood of the separation point without the existence of a singularity.The velocity field was measured using a dual-beam laser-Doppler anemometer with optical frequency shifting. The wall-shear-stress distribution was measured with a flush-mounted hot-film sensor and the wall-pressure distribution was measured using a strain-gauge pressure sensor. The various terms in the Navier–Stokes equations were evaluated directly from the measurements, permitting the validity of the boundary-layer equations to be established. Proposed solutions for the flow field are compared with the measured flow field.

Investigation on the effect of leading edge tubercles of sweptback wing at low reynolds number

2020 ◽  
Vol 21 (6) ◽  
pp. 621
Author(s):  
Veerapathiran Thangaraj Gopinathan ◽  
John Bruce Ralphin Rose ◽  
Mohanram Surya
Keyword(s):  
Leading Edge ◽  
Reynolds Numbers ◽  
Sweep Angle ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Flow Energy ◽  
Airplane Wing ◽  
Low Reynolds ◽  
Naca 0015 ◽  
Wing Performance

Aerodynamic efficiency of an airplane wing can be improved either by increasing its lift generation tendency or by reducing the drag. Recently, Bio-inspired designs have been received greater attention for the geometric modifications of airplane wings. One of the bio-inspired designs contains sinusoidal Humpback Whale (HW) tubercles, i.e., protuberances exist at the wing leading edge (LE). The tubercles have excellent flow control characteristics at low Reynolds numbers. The present work describes about the effect of tubercles on swept back wing performance at various Angle of Attack (AoA). NACA 0015 and NACA 4415 airfoils are used for swept back wing design with sweep angle about 30°. The modified wings (HUMP 0015 A, HUMP 0015 B, HUMP 4415 A, HUMP 4415 B) are designed with two amplitude to wavelength ratios (η) of 0.1 & 0.24 for the performance analysis. It is a novel effort to analyze the tubercle vortices along the span that induce additional flow energy especially, behind the tubercles peak and trough region. Subsequently, Co-efficient of Lift (CL), Co-efficient of Drag (CD) and boundary layer pressure gradients also predicted for modified and baseline (smooth LE) models in the pre & post-stall regimes. It was observed that the tubercles increase the performance of swept back wings by the enhanced CL/CD ratio in the pre-stall AoA region. Interestingly, the flow separation region behind the centerline of tubercles and formation of Laminar Separation Bubbles (LSB) were asymmetric because of the sweep.

Commissioning brownstock washing controls for an evaporator limited mill

TAPPI Journal ◽  
2016 ◽  
Vol 15 (7) ◽  
pp. 459-464
Author(s):  
RICARDO SANTOS ◽  
PETER HART
Keyword(s):  
Control System ◽  
Negative Impact ◽  
Black Liquor ◽  
Pulp Mill ◽  
Control Program ◽  
Separation Point ◽  
Water Control ◽  
Oxygen Delignification ◽  
Percentage Points

An automated shower water control system has been implemented to reduce the volume and variability of weak black liquor being sent from the pulp mill to the evaporators. The washing controls attempt to balance the need for consistent and low soda carryover to the bleach plant with consistently high weak black liquor solids being sent to the evaporators. The washer controls were implemented on two bleachable grade hardwood lines (one with oxygen delignification, one without oxygen delignification) and one pine line. Implementation of the control program resulted in an increase in black liquor solids of 0.6 percentage points for the hardwood lines. Significant foam reduction was realized on the pine line since the pine black liquor solids were able to be consistently maintained just below the soap separation point. Low black liquor solids excursions to the evaporators were eliminated. Bleach plant carryover was stabilized and no negative impact on chemical consumption was noticed when controlling weak black liquor solids to recovery.

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