Turbulence Characteristics for Flow Past a Spur Dyke on Rigid Bed Meandering Channel

Turbulence Characteristics for Flow Past a Spur Dyke on Rigid Bed Meandering Channel

Kedar Sharma

Civil Engineering Department

BML Munjal University, Gurgaon -122413, Haryana

Abstract- In the présent study tturbulence characteristics in the presence of spur dyke on rigid bed meandering channel with trapezoidal cross-section have been presented. Acoustic Doppler Velocimeter (ADV) was used to measure the velocities. The results show that, the magnitude and location of maximum turbulence intensities and Reynolds stresses changes according to the location of the spur dyke. The maximum turbulence intensities are observed along the boundary of separation zone. The turbulence intensity in longitudinal direction is higher than lateral and vertical direction component. In addition, the turbulence intensities near the bed are higher than the middle of flow depth for most of the locations..

Keywords: Channel, ADV, Turbulence Characteristics, Reynolds Stresses

1. INTRODUCTION

   Spur dykes are one of the river training structures used to protect the river banks from erosion and to deepen the main

   dissipationhave the limitations[1]. It can be observed that unlike logarithmic velocity profile, Reynolds stresses and TKE methods not require the depthwise variation of velocity. The nearbed Reynolds stresseswasusedto calculate the bedshear stresses[1, 2 and 4].

   The sediment motion and scouring in the channelisalso the function of meanbedshear stress and turbulence [5 and 7] and the sediment transport increases markedly with increasing turbulence level.

   Thus, the main objective of the presentstudyis to present the turbulence characteristics due to flow past a perpendicular spur dyke in rigid bed meandering channel.

   III. EXPERIMENTAL PROCEDURE

   The experiment has been carried out in the Hydraulics Laboratory of the Indian Institute of Technology, Kanpur. The channel center line follows a sine generated curve represented by with 0 = 500, the length of the sine-generated

   channel for creating navigational channel. Characteristic

   channel

   0 cos 2 s L

   along the channel centre

   parameters of a spur dyke are dyke length (b), height, width, shape, inclination to the downstream bank () and permeability.

   The flow field and flow separation zone due to a spur dyke in a straight rectangular channel with rigid bed have been studied experimentally and numerically by various researchers[6]. Separation zones are observed both on the upstream and downstream sides of a spur dyke[6].

   Various instruments such as Pitot-tube, micropropeller, electromagnetic current meter, Nixon 403 Streamflow-speed miniature current flow meter were used to measure the velocity field in open channel flow. The instantaneous velocity measurement was not possible by the above mentioned equipments. With the advancement of technology, Acoustic Doppler Velocimeter (ADV) and Lasser Doppler velocimeter (LDV) was used by various researchers to measure the three-dimensional velocity field. Instantaneous velocities measured by ADV and LDV are used to calculate turbulence characteristics in the channel.

   .

2. WHY TURBULENCE CHARACTERISTICS

   Originally, bed shear stresses were calculated from the slope of a linearregression fit to the meanvelocity profiles from the bedto 20 percent of the flow depth. Variousmethods for estimation of bedshear stress by logarithmicvelocity profile, Reynolds stresses, turbulent kineticenergy (TKE) and energy

   line, L = 6.2m and the wave length, = 4.65m Fig. 1[6]. Water from a constant head reservoir is supplied to the channel. A honey-comb is used at the upstream of the channel to ensure calm entrance of water. A tail gate is used at the downstream end of the channel to control the flow depth. The spur dyke used in the present study is a wooden block of height = 0.25m and thickness = 0.03m. The three-dimensional velocity field is measured with the help of a downward looking ADV attached to atraverse.

   Any space in the channel is defined by a curvilinear coordinate system (s = along the center line of the flume, n = along lateral direction and z = along vertical direction)[6]. The corresponding non-dimensional coordinates are defined as s* = s / L, n* = n / B and z* = z / Hrep, where, B = channel half width at half of the representative flow depth and Hrep = representative flow depth defined by the water surface measured at the channel center = 0.12m at s = -2.55m. The discharge is estimated by the velocity area method. All velocity measurements are recorded for 120s with a frequency of 25Hz. The cross-section being trapezoidal, b is defined as the length at z* = 0.50. The measurements are performed at two elevations i.e. at z* = 0.167 (0.02m from bed) and z* = 0.50 (0.06m from bed) and in a grid with s*= 0.0208 and n*= 0.156. In this study, all output data from ADV were processed and filtered using public domain software WinADV-version 2.027.

   The Reynolds decomposition into mean and fluctuating positions is used to analyzing turbulence velocity fields

   Table 1: Cases considered in the present study

   ( u j u j uj ). Here,

   u j

   u j (j = s, n, z)is the

   td>

   0.281

   Effect of	Case	Given conditions
   		Spur dyke location		b (m)	Urep (m/s)
   		Bank	s*
   Location	1	Right	0.125	0.160	0.225
   	2	Right (Apex 1)	0.250	0.160	0.225
   	3	Right	0.375	0.160	0.225
   	4	Right (Cross- over)	0.500	0.160	0.225
   	5	Right	0.675	0.160	0.225
   	6	Left (Apex 1)	0.250	0.160	0.225
   	7	Left	0.375	0.160	0.225
   	8	Left (Cross- over)	0.500	0.160	0.225
   Spur Dyke Length	9	Right	0.250	0.135	0.225
   	10	Right	0.250	0.110	0.225
   	11	Right	0.500	0.135	0.225
   	12	Right	0.500	0.110	0.225
   	13	Left	0.250	0.135	0.225
   	14	Left	0.250	0.110	0.225
   	15	Left	0.500	0.135	0.225
   	16	Left	0.500	0.110	0.225
   Inflow Velocity	17	Right	0.250	0.160	0.141
   	18	Right	0.250	0.160
   	19	Right	0.500	0.160	0.141
   	20	Right	0.500	0.160	0.281
   	21	Left	0.250	0.160	0.141
   	22	Left	0.250	0.160	0.281
   	23	Left	0.500	0.160	0.141
   	24	Left	0.500	0.160	0.281

   uj

   instantaneousvelocity, is mean velocity and is

   fluctuation velocity. In the present study, resultant velocity is

   defined as U

   u 2 v 2 .

   The RMS of the turbulence denotes the standard deviation of the samples taken by the Vectrino and is equal to the turbulent intensity for the respective velocity component. For example, the RMS turbulence for the s velocity component is:

   RMSu

   u2

   u u

   n 1

   n2

   The non dimensional turbulence intensities are defined

   u RMS u U

   as j

   j rep

   . The Reynolds stress is a transport

   effect resulting from turbulent motion induced by velocity fluctuations with its subsequent increase of momentum exchange and of mixing (Chanson 2008). The three Reynolds

   stress are defined as

   u'v' ,

   u'w'

   and

   v' w' . The

   corresponding non-dimensional Reynolds stresses

   (uv) u'v' /U 2 (uv) u' w' /U2

   are

   (vw) v' w' /U2

   rep ,

   rep and

   The contour of non-dimensional turbulence intensities, u+, v+ and w+ at z* = 0.50 for Case 2 (spur dyke is located on right

   rep .

   The non dimensional turbulent Kinetic energy is defined as:

   TKE 0.5(u )2 (v )2 (w )2

   1. RESULT

      The present study takes into account 24 different cases to evaluate the effects of spur dyke location, spur dyke length and inflow Froude Number on the turbulence intensity and Reynolds stresses (Table 1).

      Fig. 1: Experimental Setup

      bank at s* = 0.25) is presented in Fig. 2. Estimated turbulence intensities show that the spatial distribution of u+, v+ and w+ is almostsimilar and u+> v+ > w+. Zone of higher u+, v+ and w+ exist near the bank with spur dyke in the upstream and along the boundary of separation zone in the downstream of spur dyke. The distribution of u+, v+ and w+ in lateral direction is such that these are minimum near the bank with spur dyke, increase towards the opposite bank and attain maximum value near the boundary of separation zone. The lateral extent of w+ is smaller than the u+ and v+. The width of higher u+, v+ and w+ zone is minimum near the spur dyke and increase as the flow moves downstream.

      For various locations of the spur dyke itcanbeobserved that maximum turbulence intensity is observed along the boundary of separation zone, irrespective the location of maximum velocity. The higher turbulence intensities are observed at some distance in the downstream instead of just near the tip of the spur dyke. The magnitude of u+, v+ and w+ is higher when the spur dyke is located in the zone of higher velocity. The maximum and the minimum u+, v+ and w+ are observed for Cases 2 and 6, respectively. The maximum, u+ = 0.46, v+ = 0.38 and w+ = 0.28. The maximum and minimum influence of spur dyke on flow field is also observed for these locations. For Case 2, higher u+, v+ and w+ are observed in a small zone near the opposite bank in the downstream of Apex 2. Similarly, for Case 6, higher

      u+, v+ and w+ are observed near the opposite bank in downstream of Apex 1.

      It can be observed that u+ are greater for z* = 0.167 in comparison to z*= 0.50 at most of the locations. The location of higher u+ zone is almost same for z* = 0.167 and 0.50. However, the width of this zone is higher for u+ at z* = 0.167.

      Unlike the turbulence intensities, the higher Reynolds stress components are found along the separation zone boundary.

      The magnitude of uv is higher than uw and vw . When

      uv

      w+

      Fig. 2 : Turbulence Characteristics near spur dyke

      the spur dyke is located on the right bank, the is

      observed in the positive range. However, it is negative when spur dyke is located on the leftbank. The uw is negative for most of the locations. In comparison to uv and uw , vw is equally distributed around zero for most of the locations.

   2. CONCLUSIONS

      An experimental study was conducted in a rigid bed meandering channel to find the turbulence characteristic field due to various locations of the spur dyke.

      The main conclusions of the present study are:

      1. The maximum turbulence characteristics are observed along the boundary of separation zone.

      2. The spatial distribution of u+, v+ and w+ is almost similar and u+>v+> w+.

      3. Turbulence intensities and Reynolds stresses decreases with decrease in contraction ratio.

      Turbulence intensities and Reynolds stresses increases with increase in inflow Froude number

   3. REFERENCES

u+

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   J. Hydraul. Eng., 2005, pp. 11261135.

2. Duan, J. G., Xudong, L. H. and Wang, Q., Mean flow and turbulence around experimental spur dike. Adv. in Water Res., 2009, pp. 1717 1725.

3. Kim, S. C.et.al., Estimating bottom stress in tidal boundary layer from acoustic Doppler velocimeter data. J. Hydraul. Eng., 2000, pp. 399 406.

4. Kuhnle, R. A.et.al., Measured and simulated flow near a submerged spur dike. J. Hydraul. Eng., 2008, pp. 916924.

5. Nelson, J. M.et.al., Mean flow and turbulence fields over two-

   v+ dimensional bed forms. Water Res. Research, 1993, pp. 39353953.

6. Sharma, K., and Mohapatra, P. K., Separation zone in flow past a spur dyke on rigid bed meandering channel J. Hydraul. Eng.,2012, pp. 897890.

7. Sumer B. M.et.al., (2003). Influence of turbulence on bed load sediment transport. J. Hydraul. Eng.,2003, pp. 585596.