Subcritical Flow Research Articles

Aerodynamic noise characteristics of non-circular cylinders in subcritical flow regime

The present study experimentally investigates the aerodynamic noise from the flow around cylinders of square and equilateral triangle cross-sections at different angles of incidence ( α). The cylinder models have a side dimension of 10 mm and a span of 300 mm. The free stream velocity ( U 0) is in the range of 12–36 m/s, and the corresponding Reynolds numbers are 7.8 × 103 to 2.3 × 104, which is in the subcritical flow regime. The characteristic acoustic tones are generated at α = 30° and 45° for square and triangular cylinders. The frequency of acoustic tones linearly increases with the free stream velocity, and the corresponding Strouhal numbers are found to be in the range of 0.13–0.16. Depending on the angle of incidence, the overall sound pressure level is higher than the background noise by 4–24 dB for the square cylinder and 3–15 dB for the triangular cylinder at U 0 = 36 m/s. The highest noise level of the square cylinder is 90 dB at α = 45° and 79 dB at α = 30° for the triangular cylinder. The spectral scaling with the sixth power of the free stream velocity indicates the dipole behaviour of the acoustic tones. The mean and root-mean-square velocity profiles in the wake region characterise the noise emissions at different angles of incidence. The comparative acoustic study of the non-circular cylinders with a circular counterpart showed that the highest noise level is from the square cylinder at α = 45°. The directivity study shows that the noise level of the square cylinder at α = 45° at 90° angular location ( θ) is higher by 6.5 dB than that at θ = 30°.

Experimental analysis of combined side weir-gate located on a straight channel

Flow measurement and control in open channel system for lateral flow is important to support the system management. The flow characteristics of combined side weir-gate are complicated due to changes in flow conditions along the side weir-gate section. Experimental investigation of the flow characteristics of both weir and sluice gates is crucial for predicting the flow through a combined side weir-gate structure. Although weir-gate structures are widely used for frontal flow in hydraulic structures, the same is not true for lateral flows. In this study, 650 laboratory tests were conducted to determine the flow characteristics of combined side weir-gate for subcritical flow, and the experimental results were analyzed to determine the effect of these characteristics and weir-gate geometry on discharge capacity. Interaction factor of combined side weir-gate is a function of the upstream Froude number, the ratio of gate opening to upstream flow depth, the ratio of distance between the top of the sluice gate and the weir crest to gate opening, the ratio of weir and gate length to upstream flow depth, and the ratio of weir and gate length to main channel width. Consequently, more discharge is passed through combined side weir-gate compared to side weir and sluice gates. The empirically derived equations for the discharge of combined side weir-gate show good compatibility with the experimental data.

Three-dimensional dynamics of a cantilevered pipe conveying pulsating fluid

This paper investigates the stability and three-dimensional (3-D) nonlinear dynamics of a cantilevered pipe with an internal fluid having a harmonic component of flow velocity superposed on a constant mean value. The nonlinear equations of motion for an inextensible cantilevered pipe with the consideration of internal pulsating flow are presented. The nonlinear inertial terms in the governing equations are replaced by equivalent displacement and velocity terms by using a perturbation method. The partial differential equations are then transformed into a set of ordinary differential equations (ODEs) by using the Galerkin method. The instability regions of the subcritical and supercritical resonances of a linear system are determined via the Floquet theory. The effects of mean flow velocity and mass ratio are investigated. The resulting coupled nonlinear differential equations are numerically solved using a fourth-order Runge-Kutta integration scheme for the subcritical and supercritical flow velocities. The nonlinear dynamical responses are presented in the form of bifurcation diagrams, time histories, phase portraits, power spectral densities (PSDs) and Poincaré maps. Some interesting and sometimes unexpected results have been observed with different flow velocities. The analytical model is found to exhibit rich and variegated dynamical behaviors which include 2-D or 3-D periodic, quasiperiodic and chaotic motions. The convergence analysis of the number of truncating modes in the Galerkin approach is also conducted.

Effect of Orientation and Vegetation over the Embankment Crest for Energy Reduction at Downstream

Coastal embankments often collapse due to the tremendous destructive energy of an overtopping tsunami flow due to a deep scour by nappe flow. Hence, to clarify the nappe flow formation condition due to the overtopping, a series of tests were carried out within a laboratory flume with immobile settings by lowering the downstream surface angle of an embankment model while keeping the upstream surface slope constant (1:1) with five non-dimensional overtopping depths and six different crest conditions. The conditions imposed on the embankment crest in the flow direction were without vegetation; horizontal crest, (−)4% descending crest slope, (+)4% ascending crest slope, and adding vegetation model with three different densities across the horizontal crest to improve resistance to the flow. The increased resistance provided by the vegetation models were categorized based on the spacing ratio between cylinders to diameter: sparse, intermediate, and dense. Increased vegetation density above the crest results in a significant reduction of flow energy by approximately 30–50% at the downstream brink edge and 40–60% at the downstream plunge basin. In contrast, the maximum energy reduction was found to be by the dense vegetation model. Additionally, owing to the steep slope of the water surface profile and the increasing vegetation density, the impinging jet’s impact point moved closer to the toe of an embankment. This implies that vegetation covers a smaller area while increasing density to mitigate the destructive intensity of flood/tsunami movement. Meanwhile, the descending crest scenario results in a faster nappe flow formation. In contrast, the ascending crest scenario delays the nappe formation while reducing the downstream slope angle. It maintains the sub-critical flow at the crest, except near the downstream brink edge.

Spanwise wake structure of a yaw-oscillating circular cylinder at subcritical flow

An experimental study is carried out to characterize the spanwise variation of the near wake of a cylinder yaw-oscillating about its mid-span between the yaw angles of θ=0o (unyawed case) and 30o at two subcritical Reynolds numbers, Re=5×103 and 1.5×104. For these tests, a cylinder with length-to-diameter ratio of 13 is immersed into a water channel, leaving a free end at the bottom end and the free water surface at the top. The yaw oscillation frequencies, when expressed non-dimensionally in terms of the reduced frequency (K), have the values of K=0.5 (low), K=1.3 to 2 (moderate), and K=4 (high). Planar Particle Image Velocimetry measurements are performed in various planes that are parallel and orthogonal to the spanwise axis of the cylinder’s unyawed version. For a cylinder undergoing yaw oscillation, the near wake is found to be highly three-dimensional. The spanwise variation in the near wake increases substantially as the reduced frequency K is changed from low to moderate values, while at high K, the flow becomes relatively uniform over large spanwise distances as a result of the delay in flow response. The spanwise flow topology is mostly independent of the Reynolds number in the subcritical range considered. The cylinder’s direction of motion, its acceleration/deceleration, and the axial flow developing as a result of large yaw are identified as factors impacting the flow behavior. For low reduced frequencies, the cylinder’s direction of motion plays the most prominent role in the topology of the near wake until large yaw angles. This effect either weakens or strengthens the reverse flow velocity and the mean recirculation region depending on the direction in which the cylinder moves. At large yaw angles, high-magnitude axial flow originating from the bottom free end develops over larger spanwise sections reducing the reverse flow velocity in the wake and eventually leading to the suppression of the mean recirculation region. For moderate values of reduced frequencies, the acceleration/deceleration of the cylinder becomes another factor influencing the flow behavior. During the phases of oscillation where the cylinder is accelerating, the impact of the cylinder’s direction of motion on the near-wake flow intensifies. In contrast, the same impact is weakened when the cylinder decelerates. At very high reduced frequencies, the mean recirculation region is suppressed over most of the cylinder’s top or bottom half span under the dominant effect of the cylinder’s direction of motion. At such high reduced frequencies, the response of the flow to the fast oscillation motion of the cylinder is delayed, revoking the impact of acceleration/deceleration of the cylinder.

Mechanism for turbulence proliferation in subcritical flows

The subcritical transition to turbulence, as occurs in pipe flow, is believed to generically be a phase transition in the directed percolation universality class. At its heart is a balance between the decay rate and proliferation rate of localized turbulent structures, called puffs in pipe flow. Here, we propose the first-ever dynamical mechanism for puff proliferation—the process by which a puff splits into two. In the first stage of our mechanism, a puff expands into a slug. In the second stage, a laminar gap is formed within the turbulent core. The notion of a split-edge state, mediating the transition from a single puff to a two-puff state, is introduced and its form is predicted. The role of fluctuations in the two stages of the transition, and how splits could be suppressed with increasing Reynolds number, are discussed. Using numerical simulations, the mechanism is validated within the stochastic Barkley model. Concrete predictions to test the proposed mechanism in pipe and other wall-bounded flows, and implications for the universality of the directed percolation picture, are discussed.

Can Lava Flow Like Water? Assessing Applications of Critical Flow Theory to Channelized Basaltic Lava Flows

AbstractFlowing lava and water have dramatically different physical properties but can form similar hydraulic structures, including undular hydraulic jumps, or standing wave trains. In water flows, undular hydraulic jumps are evidence of critical flow (Froude number ∼1) and open‐channel hydraulic theory provides a powerful tool for estimating flow depth and velocity. Monitoring these parameters in an active lava channel is inherently challenging, but essential for calculating lava discharge (effusion rate), a primary control on the rate of flow front advance and ultimate flow runout distance. We analyze undular hydraulic jumps in both water and lava flows to assess the conditions under which they form and, by extension, the potential use of critical flow theory to estimate, in real time, lava flow velocity, depth, and discharge. Experimental data for water flows show that these structures mark the transition from supercritical to subcritical flow. Undular hydraulic jumps in the near‐vent lava channel of the 2018 lower East Rift Zone eruption of Kīlauea, Hawaiʻi also reflect critical flow conditions; their wavelengths scale with flow depth and velocity, consistent with hydraulic theory. Calculated lava effusion rates are similar to estimates made using more traditional approaches (Jeffreys', 1925, https://doi.org/10.1080/14786442508634662, equation based on lava viscosity, density, and channel slope) and with lava volumes derived from topographic‐change mapping. From this we conclude that critical flow phenomena show great potential to track flow dynamics and inform hazard assessment for a wide range of geophysical fluids.

Studying the influence of Constricted Froude’s Number on Energy Dissipators in Open Channel Flow

Model experimentation was used to examine the flow properties above the perpendicular descents with a grid roof, or the “grid drop-type dissipators”. The software ANSYS FLUENT R 15 is used to analyse the fluid flow above the impediment and research of the flow arrays and hydraulic characteristics. The purpose of the flow analysis is to examine how subcritical flow develops upstream and to let the water flowing downstream to slow down the flow of water to prevent erosion downstream. The pool formation, mixing zone, pressure distribution, and velocity distribution were all analysed using the Ansys software. The findings show that, in comparison to a simple vertical drop, the suggested hydraulic structure removes undesirable flow situations and serves as the foundation for a more operative flow rheostat arrangement. It is shown that, in comparison to a straight vertical drop, the scattering/circulation process in the pool surges the velocity grade, the impetus altercation, and, as a result, the energy dissipation.

Experimental Investigation on Hydraulic Efficiency of Vertical Drop Equipped with Vertical Screens

In the present study, vertical screens were utilized at downstream of vertical drops to increase the energy dissipation of subcritical flow. The experiments were carried out using screens with two different porosity ratios (40% and 50%) and three different distances from the drop brink (30, 60 and 90 cm). The results reveal that drops equipped with screens increase the relative downstream depth, the relative pool depth, and the relative energy dissipation compared with a plain vertical drop. By increasing porosity ratios and the screen distance from the drop brink, the relative downstream depth and relative energy dissipation increase, whereas the relative pool depth decreases. Also, by increasing the relative critical depth, the relative energy dissipation of the vertical drop decreases, whereas the energy dissipation related to the screens increases. However, increasing the relative critical depth initially increases and then decreases the performance of the hydraulic jump in terms of total energy dissipation.

Investigation of Blockage Upstream Box Coverage on the Scour Pattern in the Open Channel

Under sub-critical flow conditions, the presence of blockage (solid wastes and weeds) upstream of box coverage and the scours pattern downstream of the coverage was explored in this study. The upstream blocking was simulated using 48 runs in an artificial trapezoidal cross-section with three square box coverages of side dimensions 8.80, 10.40, and 12.90 cm, four water flows of 2, 5, 8, and 11 L/s, and four blockage ratios of 0, 10, 20, and 30% relative to the coverage cross-section area. To estimate the scour hole characteristics, a 2.00 m long, 0.60 m bed width, and 0.30 m deep sand basin filled with D50 = 0.50 mm bed material was constructed directly downstream of the coverage outlet. In each run, the water level, velocity, and scoured hole parameters downstream of the coverage were measured. According to the analysis, the Non-blocked coverage has less scour depth and length than the partially blocked coverage, where the maximum scour depths and length of the Non- blocked coverage for cases 1, 2, and 3 at the discharge of 11 l/s were 73, 72, and 17 % respectively, and 77.56, 77.34, and 83.66 % respectively relative to the maximum scour depth and length of partially blocked coverage at the discharge of 11 l/s and blockage ratio 30 %. The depth and length of the scour hole downstream coverage are increased with the increment of the coverage's blockage ratio and discharge and are reversely proportional to the inlet area of coverage. Increased relative scour depth and length by 3.60 percent and 11.80 percent respectively, by increasing the downstream Froude number of flow (Frd) by 0.01 while the coverage’ relative wetted area (Ar) is constant. The study suggested applying coverage with a suitable area and water discharge, and protection techniques downstream of the coverage to reduce the influence of coverage and blockage on the open channel's hydraulic efficiency and the scour pattern downstream of the coverage. Also, install a trash rack upstream of the covering and remove aquatic weeds and solid wastes periodically upstream and inside the coverage.

Effect of the flow regime on the hydraulic features governing the operation of vortex drop shafts with spiral inlets.

If the operation of existing vortex drop shafts should be verified, then it is essential to know the hydraulic performance of these special structures under both subcritical and supercritical flow regimes. The purpose of the present research consisted of providing practical guidelines and recommendations for managing the hydraulic design and verification of subcritical and supercritical vortex drop shafts. The examination of various experimental results from physical model investigations allowed to show that the inlet channel and the spiral inlet behaved differently depending on the energy approach flow content. The main dissimilarity lay, however, in the functioning of the vertical shaft and the dissipation chamber. The rotation of the falling flow along the vertical shaft was more evident for approaching supercritical flows. Severe flow conditions in terms of water depths and bottom pressures could be observed in the dissipation chamber under a supercritical flow regime. The design of this special component must be carried with prudence compared with the subcritical flow regime because failure events as the chamber submergence and the crash of the bottom surface just under the shaft outlet may occur for approaching supercritical flows.

An experimental modal testing method for subcritical flow around a cylinder

Modal analysis of fluid flows is essential for understanding flow physics and fluid–solid interaction mechanisms and for implementing flow control. Unlike unstable flows, the intrinsic stability of subcritical flows has led to failures in experimentally extracting the clear structure of the subcritical flow modes. To this end, this paper proposes an experimental modal testing method for subcritical flows. Using dynamic mode decomposition, dominant modes of flow around a cylinder at subcritical Reynolds numbers are extracted experimentally for the first time. The extracted structure and parameters of the modes are in agreement with the numerical results in the literature. It is found that the first-order mode is the stable von Kármán mode and can be observed at a Reynolds number as low as 19.3, which is nearly identical to the lower boundary of subcritical vortex-induced vibration. This finding provides the first experimental evidence of the correlation between the von Kármán mode and fluid–solid interaction instability in subcritical flows.

Study of hydraulic characteristics of trapezoidal piano key side weir using different approaches

Abstract Side weirs are widely used in hydraulic engineering applications. The studies on the subject have been generally focused on classical and labyrinth side weirs. However, the same is not true for piano key side weirs (PKSW) in a straight channel. The piano key weir (PKW) has high discharge capacity compared with classical weirs. In this study, the hydraulic characteristics of a trapezoidal piano key side weir (TPKSW) in straight channels were investigated experimentally. In all experiments, the hydraulic characteristics of nine TPKSW models were studied extensively using the De Marchi, Domínguez and Schmidt approaches in the subcritical flow regime, with Froude number range 0.12 < F1 < 0.87. The results show that a TPKSW provides better performance compared to traditional rectangular and triangular labyrinth side weirs. Specifically, for the 0.12 < F1 < 0.4 condition, the efficiencies of a TPKSW and trapezoidal labyrinth side weir are close to each other. A trapezoidal labyrinth side weir is more efficient than a TPKSW at larger Froude numbers. The discharge capacity of the TPKSW is 2.9 to 12 times higher than that of the rectangular side weir. Scatter diagrams were obtained for CPW and F1 numbers using various approaches available in the literature. The diagram generated by the De Marchi approach has much less scattering, compared to the diagrams generated by the Domínguez and Schmidt approaches. It has been determined that TPKSWs are an effective type of side weir in lateral flows. Lastly, an empirical equation was obtained for the discharge coefficient, which is in good agreement with the experimental data.

Simulation of Accelerated Subcritical Flow Profiles in an Open Channel with Emergent Rigid Vegetation

Even though both fluid mechanics and numerical studies have considerably progressed in the past decades, experimental knowledge remains an important tool for studying the resistance to flow in fluid media where a complex environment dominates the flow pattern. After a comprehensive review of the recent literature on the drag coefficient in open channels with emergent rigid vegetation, this paper presents the results related to 29 experimental accelerated subcritical flow profiles (i.e., M2 type) that were observed in flume experiments with emergent stems in a square arrangement at the University of Calabria (Italy). First of all, we used some of the literature formulas for the drag coefficient, concluding that they were unsatisfactory, probably because of their derivation for uniform or quasi-uniform flow conditions. Then, we tested a recently proposed approach, but when we plotted the drag coefficient versus the stem Reynolds number, the calculated drag coefficients showed an inconclusive behavior to interpret. Thus, we proposed a new approach that considers the calibration of the Manning coefficient for the simulation of the free surface profile, and then the evaluation of the drag coefficients based on the fundamental fluid mechanics equations. With the help of classical dimensional analysis, a regression equation was found to estimate the drag coefficients by means of non-dimensional parameters, which include vegetation density, stem Reynolds number and flow Reynolds number computed using the flow depth as characteristic length. This equation was used to simulate all the 26 observed profiles and, also, 4 experimental literature profiles, and the results were good. The regression equation could be used to estimate the drag coefficient for the M2 profiles in channels with squared stem arrangements, within the range of vegetation densities, flow Reynolds numbers and stem Reynolds numbers of the present study. However, in the case of the three profiles observed by the authors for staggered arrangement, the regression equation gives significantly underestimated flow depths.

Systematic Route to Subcritical Dynamo Branches.

We demonstrate that the nonlinear optimization of a finite-amplitude disturbance over a freely evolving and possibly even turbulent flow, can successfully identify subcritical dynamo branches as well as the structure and amplitude of their critical perturbations. As this approach does not require prior knowledge of the magnetic field amplification mechanisms, it opens a new avenue for systematically probing subcritical dynamo flows.

Characteristics of runoff and infiltration processes on slope with intergrown biocrusts and herbs

Biocrusts and herbs coexist in arid and semi-arid areas, playing an important role in soil and water conservation. However, the combined effects of biocrusts and herbs on process and mechanism of runoff and infiltration on slope are still unclear. In this study, simulated rainfall experiments with four treatments, including bare soil, biocrusts, Stipa bungeana and S. bungeana+biocrusts, were designed to investigate runoff, infiltration process and hydrodynamic characteristics of herbs and biocrusts on slope. The results showed that the runoff under the two treatments with biocrusts was stable. The total runoff of four treatments was in the following order: biocrusts > S. bungeana+biocrusts > bare soil > S. bungeana, indicating that biocrusts could inhibit soil infiltration and S. bungeana could promote soil infiltration. At soil depths of 16 cm and 24 cm, the cumulative infiltration of biocrusts treatment was significantly less than that of S. bungeana+biocrusts treatment, suggesting that with the increases of soil depth, S. bungeana reduced the negative effect of biocrusts on soil infiltration. The Froude number was greater than 1 under bare soil and less than 1 under other treatments, which was subcritical flow. Compared with the runoff kinetic energy of the bare soil treatment, the treatments of biocrusts, S. bungeana, and S. bungeana+biocrusts were reduced by 83.3%, 59.5%, and 88.1%, respectively. The variations of hydrodynamic parameters indicated that the role of biocrusts is greater than S. bungeana in regulating runoff.

Dynamics of a buoyant gravity current propagating in a linearly stratified medium

In this study, we investigate partial- and full-depth buoyant gravity currents propagating along the top surface in a linearly stratified medium. Two- and three-dimensional numerical simulations are performed to study the effect of stratification and initial current depth, on the front speed, internal wave field, and turbulence characteristics. The stratification is varied through a non-dimensional parameter R=ρ0−ρCρb−ρ0, ranging between 0.04 and 85, where ρC is the constant bulk density of the current fluid and ρ0,ρb represent the densities of the ambient fluid at the top and bottom surfaces, respectively. For large values of R (ρ0−ρC≫ρb−ρ0), we observe that the resulting Froude number (Fr=UNH) is greater than 1/π, and the flow is characterized as supercritical, where the front speed exceeds the long wave speed. In the supercritical regime, Kelvin–Helmholtz billows are prominently seen along with an internal solitary wave, which propagates with the density front. As the R value decreases, the relative strength of the ambient stratification increases when compared to the horizontal density difference at the top surface, leading to a subcritical flow regime where the front speed is smaller when compared to the long-wave speed. The Kelvin–Helmholtz billows and the solitary wave gradually disappear, and vertically propagating high-mode internal waves are prominently seen for R< 1. Quantification of the Froude number for various values of R and h/H shows that it follows a power law, Fr∝(h/H×R)1/2, with the proportionality constant 0.72. This scaling works well for all the partial-depth cases considered in this study, i.e., h/H= 1/8, 1/6, 1/4, and 1/3, while a slight deviation is observed for the full-depth gravity currents that correspond to h/H= 1.

Self‐Aeration of Supercritical Water Flow Rushing Down Artificial Vegetated Stepped Chutes

AbstractVegetation flow is commonly studied for subcritical flow motions, while in supercritical flow, the presence of vegetation often leads to enhancement of flow self‐aeration, which would in turn alter the flow hydraulics and water‐plant interaction. This paper presents an experimental investigation of aerated water flow cascading down a vegetated stepped chute. The 21.8° sloping chute is equipped with a 12‐m‐long uniformly stepped invert section. The 0.12‐m‐high steps have grate‐shaped rough top surface covered by 0.07‐m‐high flexible artificial plants, and three vegetation densities are tested in skimming flows, in addition to the reference cases of smooth steps and rough steps covered by bare grate only. The air‐water flow properties and free‐surface fluctuations are systematically measured from the upstream clear‐water region to the downstream fully developed aerated region. The experiment results demonstrate that, compared to a man‐made concrete stepped chute, the presence of vegetation cover on the steps reduces the overall self‐aeration and unsteady fluctuating motions of the flow, while it enhances the dissipation of the flow kinetic energy. The present finding also suggests that the modification to flow aeration and energy loss in the vegetated flow over macrostep‐shaped roughness is dependent on the flow regime which is primarily determined by the flow rate for a given slope and step‐induced macroroughness height.

MODIFIKASI KOLAM OLAK TIPE AMBANG BERGIGI DALAM RANGKA REHABILITASI KERUSAKAN BENDUNG CIKEUSIK DI JAWA BARAT

The presence of hydraulic jumps downstream of the weir structure is a natural occurrence that indicates a change in supercritical to subcritical flow characteristics. Hydraulic jump is one indicator of the dissipation of energy in the stilling basin. Therefore, the absence of a hydraulic jump can be a problem for some weirs, as happened in the Cikeusik Weir, which experienced damage to the stilling pool floor so that the energy dissipation did not function optimally. This paper will discuss the potential causes of damage to the Cikeusik weir and alternative designs for its countermeasures. A physical model test was carried out in the laboratory with a scale of 1: 40 to obtain an appropriate alternative to the stilling pond design for the Cikeusik weir. The tests were carried out with variations in the discharge Q2 years, Q25 years, and Q100 years, respectively. Three model tests were conducted, namely the existing and two modified designs. Based on the model testing results, extending stilling basin floor length with a dentated end sill and adding downstream riprap is the most optimal alternative because it has an energy reduction effectiveness of up to 80% and the lowest downstream scour potential compared to other options. The results of this test are expected not only to be used as an alternative to mitigate the damage to the Cikeusik weir but also to other locations that have similar problems. Keywords : Cikeusik Weir, stilling basin, Froude number, local scour, physical model

On the High Winds in the Tianshan Grand Canyon in Northwest China: General Features, Synoptic Conditions, and Mesoscale Structures

This work studies the characteristics of high winds in the Tianshan Grand Canyon (TGC), Northwest China by using surface wind observations at six automatic weather stations during 2017–2018. Three high wind indices are examined, namely, high wind hour (HWH), station high wind event (SHWE) and regional high wind event (RHWE). SHWE denotes persistent high winds of more than 3 h at an individual station while RHWE is defined for the southern TGC as a whole. High winds are mainly northwesterly/westerly, occurring predominantly in southern TGC. HWHs most often occur in spring and summer, while winter and spring HWHs possess the strongest intensity. The occurrence of HWHs exhibits an apparent diurnal variation, most frequent in the afternoon and evening while least from mid-night to early morning. SHWEs and RHWEs are prone to take place in spring, possessing relatively long lifetime and strong intensity. According to composite analysis of the 0.25° FNL reanalysis, favorable synoptic conditions are obtained for the persistent high winds in spring. Of great importance is the mid-tropospheric trough in the mid-high latitudes located to the north (i.e., upstream) of the TGC. The cold advection behind the trough cools and stabilizes the mid-lower troposphere, while the post-trough sinking motion causes a surface high that accelerates the low-level northwesterly wind. The synoptic thermal and dynamical forcing help set an upstream flow of moderate Froude number such that the mesoscale orographic flow traversing the TGC is in the nonlinear high-drag regime. Large-amplitude gravity waves are excited, with the isentropes descending abruptly over the lee slope which resembles internal hydraulic jump. Downslope windstorms are produced at the expense of flow potential energy as the subcritical upstream flow is transitioned to supercritical at the TGC peak. These findings have important implications on the formation and prediction of high winds in the TGC.