Monday, March 17, 2014

Combined Sewer Overflow (CSO) hydraulics

Today's post is about a simulation for a municipal application. I usually draw on work-related inquiries for these blog posts, and this week a municipal design consultancy called asking about how we handle the combined sewer overflow problem (heads up the answer is very well!). Typically there are two important aspects, one is understanding the straight hydraulics in both a transient and a steady state sense, the other is the characterization of particulate dynamics in the flow stream. In this case, the interest was purely on understanding the hydraulics under various upstream water elevations and pressures, in both free stream and fully pressurized conditions.

Geometry of interceptor sewer with regulator vault access in
the far back. Flow enters from left, all walls not shown.
The 'lay of the land' here is that we have an interceptor sewer carrying the flow under potentially free surface or pressurized conditions. A T-junction allows some of the fluid to make its way to a regulator vault which can then discharge some of the fluid into a separate culvert through a vertical drop-shaft.

To clarify the geometry I created a rotating animation so that you can see the inflow, the regulator vault and the drop shaft where some of the fluid exits into an underlying culvert from different viewpoints:

I ran two scenarios. In the first scenario we begin with a stagnant low level of fluid in the main sewer, then adjust the inflow boundary condition with time to simulate a rising water level. Initially this only increases the flow rate and fluid elevation in the interceptor sewer. At a given height the water begins to 'overflow' into the regulating vault section of the structure. The fluid then exits into the culvert.

This animation shows some additional filling detail:

In the second scenario, we restart the simulation once the fluid occupies the full volume of the interceptor sewer, and then continue to increase to upstream pressure: the sewer interceptor now operates under pressurized conditions. What is interesting to observe here is that while the sewer interceptor chamber now runs under fully pressurized, confined flow conditions, the regulator vault continues to operate under free surface conditions due to the limiting effect of the regulating slide gate.

Finally, although this wasn't asked for, it is noteworthy that particle tracking can be included in the analysis. FLOW-3D can assign particle sources of different sizes and densities and fully couple their behaviors with the hydraulics of the flow. We can use particles as tracers of course, but more dynamically sophisticated behaviors are equally straightforward to implement.

As always I encourage you to sign-up (top right of the page) so that you will be notified of future posts!

I leave you with a link to an interesting read on a related topic,"Testing a Complex Hydraulic Design of a Sewer Transition with FLOW-3D. Comparison with a Physical Model", by Daniel Valero, Rafael García-Bartual, Ignacio Andrés and Francisco Valles of the Polytechnic University of Valencia.

Hydraulic Design of a Sewer Transition

cheers all

john wendelbo

Monday, February 24, 2014

Thermal Plume Analysis using FLOW-3D

FLOW-3D Model Setup - Two power-plant outflow ducts
are located near the power-plant on the left bank.
Hello again. I received an inquiry this week about our ability to model the near field structure of thermal plumes, generated by the discharge of warm waters from power plant cooling systems.

The framework of this request was the EPA 316(a) and (b) set of regulations, and in fact these modeling methods for thermal plumes are very closely tied to those used to evaluate the hydraulic zone of influence (HZI) of Cooling Water Intake Structures (CWIS). You can learn more about modeling HZIs by downloading this webinar which presented last December (YouTube link here if you want the audio!)

Getting back on topic, FLOW-3D resolves the Navier Stokes equations including detailed free-surface dynamics, in 3D and with a number of advanced turbulence models available, and also fully couples density changes (via temperature changes, varying initial conditions, or changes in salinity, turbidity, etc). In addition, spatial scales of tens of kilometers upstream or downstream nowadays are very efficiently handled in high detail, no need to simplify anything down to 1D or 2D models, certainly not on the basis of computational time, that rational simply no longer exists.

"Lay of Land" - Bathymetry and geospatial layout of the power-plant, ducts, bridges and weir.
I might add that what I have witnessed (over and over from running similar simulations) is that these flows, in particular the thermal structures (or indeed upstream entrainment probability maps) are in fact highly three dimensional: the vertical structures and distributions of flow parameters are anything but uniform through the water column, and the detail of the bathymetry is very significant. My take home message here is to use a 3D solution in order to model these phenomena accurately.

This said, even if the flow characteristics turn out to be complex, these types of simulations are actually quite straightforward to set up in FLOW-3D. In particular our free gridding approach to meshing fluid and solid quantities makes not only the setup, but also subsequent modifications in the parameter space (such as flow conditions or new geometric configurations) extremely efficient: whether it's the initial meshing of the problem, or subsequent iterations, the meshing operations are in all cases minimal.

Thermal Plume Map, in the fluid (left) and on the river bed (right indents)

In this example the bathymetry represents a junction in the San Antonio River, and I added some detail (the power plant, the exhaust ducts, exhaust volume flow rates and densities), and turned on the additional physics that relate to thermal plumes (density evaluation, turbulence model, and mass sources), not much to it!

Typically then we would look at the three dimensional temporal and spatial distributions of the fluid density or temperature (in this case I used density as a proxy), and then maybe would take some 2D plane cuts to get a sense of what regions are affected and when.

What I found really interesting is that FlowSight (FLOW-3D's next generation post-processor) allows you to color components (rather than fluids, i.e., the river bed itself) by fluid property on the contact surface.

What this means is that we can very easily map the regions of the river bed which will be affected by the thermal plume. If you are looking at benthic communities, this is the way to do it!

Here is an animation of the result of this simulation. If you click on the little full screen icon (bottom right of the video) you will get a better view of what is going on. The right hand side window is a view of the fluid with the exhaust's thermal signature, as is the top left window, the middle left is a 2D vertical plane cut just downstream of the bridge, and the lower left window is that view I mentioned, where we see the thermal signature on the river bed itself, as opposed to in the fluid.

In conclusion, if mapping thermal plumes is your business, I would say FLOW-3D is a pretty phenomenal tool to conduct such studies, for its accuracy, robustness and speed, but also for the ease of use and simplicity in model setup, and of course the ease with which one can visualize and analyze results!

Cheers all, until next time, any questions feel free to write directly.

John Wendelbo

Sunday, February 9, 2014

FLASH FLOOD Simulation using FLOW-3D

Topography used in this study, the top layer is porous,
the lower layer is bedrock
Today I found myself curious about flash floods. As FLOW-3D users already know we usually do very well for extreme event modeling, complex free surfaces, etc. Oddly enough the first question was, could we model precipitation? Historically our code hasn't particularly been used for flood plain/watershed type problems. I presume mostly because 2D solutions work just fine for the most part, and so I took a morning out of my week to sort this out, the result I thought was exhilarating! In the case of flash floods my conclusion is we do it very well (and are very effective computationally).

The two main models used here are the mass source and the porous media models.

Initial condition before rainfall*
The way this simulation is set up is that we have a topography with an upper layer which is a porous media, this layer overlaps a bedrock which is impermeable. In addition the upper layer is defined as a source of fluid on its upper surface. In this simulation I am modeling a flash flood with intense precipitations lasting 180 minutes. The lower layer is bedrock, and as expected initially, the flow is governed by both seepage into the upper, permeable layer and the usual advection/Navier Stokes behaviors. Once the permeable layer is saturated, we start seeing more of the surface water dynamics, which in turn turn lead to torrential flows down in the lower elevations of the canyons.

Intermediary stage, the ground is still
mostly soaking water
Valuable, actionable information is easily visualized as a result of the effort. In this case we simulated two situations concurrently: the run-off and subsequent torrent formed in a canyon (to the right of the domain), and the run-off from mini-canyons into an adjacent pre-existing body of water (to the left of the domain).

The precipitation map can be fully defined both spatially and temporally across the entire domain, and porous behaviors can be modeled both for saturated and un-saturated media.

Torrent formations following heavy precipitation. Note the water pooling and main torrent formation in the canyon on the right, and the strands of heavy discharge into the pre-existing body of water on the left.
Here is the animation of the event:

You can also view the video on youtube here.

Finally it's very interesting to look at the time series of water depth located at a probe placed near the lower end (elevation wise) where I was expecting to see the great volume flow and water pooling. Initially, while the porous layer is actively absorbing water, we see no surface water pooling. Then once the layer becomes saturated, we observe a rapid increase in water depth, until we finally observe the steady state balance between the influx of precipitation and the outflow via the torrent.

Finally I note that you can be a lot more sophisticated with the mapping of the precipitation rates by simply tiling your domain into various sub-components (they can be of arbitrary shape by the way):

Cheers all, don't forget to sign-up for email updates (top right of this page) if you want to be notified of future posts!


*All simulations post-processed with FlowSight, coming soon in FLOW-3D v11

Type II spillway modelization

As customary for a first blog post, hello world!

This spillway was designed following USBR guidelines (type II), where the engineers were interested in validating their designs without having to resort to physical lab models (expensive, and which also have their own sets of scaling problems).

The steps to analyze these types of problems in FLOW-3D are straightforward:

Post-processed with FlowSight, coming soon in FLOW-3D v11
  • Generation of a 3D CAD model based on the design blue prints (I happen to use Rhino3D, works pretty well for me)
  • Geometry import in FLOW-3D in the STL format
  • The usual FLOW-3D simulation steps:
    • set the boundary conditions
    • set the initial conditions
    • choose your physical models (in this case turbulence and air entrainment)
    • choose your outputs (hydraulic quantities, Froude #, etc.)
In the case of spillways, which have fairly large spatial scales compared to the water depth, it is EXTREMELY advantageous to use the code's domain removing capabilities. The best way to do this is to follow a 4 step approach:
  1. Run the simulation with a coarse grid. This run will take 10 minutes at most, and even if this won't catch the full manifestation of the hydraulic jump, it will be enough to give you a good idea of where the fluid regions will be active.
  2. Equipped with that information go back into your CAD package and create a domain removing solid which conforms reasonably well with the complement of your fluid form (my method is to super-impose a picture of the simulation result within my CAD model).
  3. Add this new solid component to your simulation as a domain removing component.
  4. You can now massively crank up the resolution of your simulation to fully capture the detail of the energy dissipation mechanism in good detail. 
You can watch the video on youtube here!

cheers all